Indexing Strategy

What do I care about when I’m playing with indexes? That’s easy. I want as few indexes as possible efficiently referenced by as many pertinent, well-tuned, consistently written queries as is reasonable. It’s explaining that last sentence that’s the hard part.

The thing that will jump out to most people is that my goal doesn’t mention a specific, single query that you want to run great.  Although that’s something I hope to achieve, it only becomes a priority as a last resort.  I’m more concerned with already having that data in memory because the index is being used by many queries, and also having fewer indexes to make data modifications more efficient. There’s more to it than that, but the detail belongs in the body of this post, not the intro.

If I was writing how to make a perfect index for a single reference to a table in a single query, this post could be done rather well in a couple paragraphs. Even though I’m focusing only on OLTP (ruling out columnstore indexes), in 99.999% of environments (ruling out in-memory hash indexes), and not getting into details of filtered indexes or indexed views, there’s still a lot to consider to the point that the first few paragraphs will just be what we’re going to keep in mind. I hope you didn’t have other plans today…

Does this advice apply to you?

It depends! Just kidding, I hate that (non) answer.

There are two targets audiences for this.  While it’s useful to everyone, you’d have to hit both of them for this to be perfect for you. First target is the person; this is written for someone who is comfortable working with indexes for single queries but wants a better view of the big picture. Second target is the database, which is a rather typical in-house OLTP database.

  • More data than you have memory
  • Writes throughout the day, especially in your larger tables
  • Read and write performance matter more than disk space
  • No extreme write loads, such as truncating and repopulating your largest table (easy fix, disable and rebuild your indexes around this action)
  • You have the ability to make indexing changes (this goes beyond what you can typically do with vendor databases)
  • Hopefully, you have the ability to make some code changes

If you or your database aren’t the perfect audience, don’t worry about it.  The most important things to know are what to keep in mind and how those things are interconnected.  The real goal is making more intelligent decisions for your databases, not fitting into a specific mold.

PreReqs!!!

Because this is an advanced look at the fundamentals of indexing strategy, you shouldn’t take offense if you have to do some prerequisite work for all of this to come together. If I give you a severe headache consider reading this stuff first, and the book on the list is well worth a second read cover-to-cover.

PreReqs:

Even with that I’ll probably still give you a headache (a common theme for me), and you’ll probably have questions. Keep in mind that some of the greatest compliments you can give someone are asking them a question and offering them large sums of cash. Although I don’t hand out my LLC’s address to send checks to unless I’ve actually looked over your indexes, I do have a free comments section below that I’d like to see used more often.

Something to consider

Here are all the things we’ll be considering. This is a great list, but nothing is ever going to be 100% all-inclusive or perfect.

Tune your queries: The ideal set of indexes for a poorly written query may be 100% different than the same query after it’s refactored.  We don’t want to constantly tinker with indexes, so this process is best if you tune your most expensive queries first.  An old, unpublished concept for this post had creating an index as a 13 step process with steps 1-11 avoiding indexes through tuning, step 12 making an index, and step 13 seeing if you could delete any other indexes.  That really wasn’t too different from Brent Ozar’s (b|t) Be Creepy method.  Indexing is not the only answer, and not the first answer either.

Query Importance: Some queries just need to complete, some need to run decently well, and some need to run as close to instantly as possible. Knowing how critical your query is will weigh in on how you index, but none of them, not even the most critical queries, will have their performance be the only deciding factor on what indexes you should have. Some outliers will require you to go back and create a specific index for them as a last resort, but there’s no reason to concern ourselves with last resorts before we get started.  If an index would work, even somewhat inefficiently, and it’ll already be in cache, do we want to create another index that will fight it for space in cache?  Don’t create another index to get your most critical query to 0.1 seconds when it’s running at 0.2 seconds and the business is happy with anything under 1.0 second.

Query Consistency: Are you querying the table the same way in all of your queries? If not, do you know you are stressing me out? Do you not care?  Using the same columns to join where possible matters, even if you could live without one of them in some cases because consistent queries mean index reusability.

Query Frequency: Some queries run five times a second, some run once a year. They aren’t even close to the same, and shouldn’t be treated the same.

Query Timing: It is different from frequency. Once-a-day is not just once-a-day. An “8-5 is critical” shop where all your users are sleeping at 3 AM means that we care much less about collateral damage from a 3 AM query. If we do a table scan on a huge clustered index at 3 AM that kicks everything out of cache it might not matter much, but do that same thing at 3 PM and we may want to consider an index even if it’s only used for a single query.

Query Justification: That query timing example threw up a red flag to me. Do we need to run that query at all? Does it need to run in prod, or is there a reporting database I can run it against? Should I consider making a reporting database? Does it need to run at 3 PM? Question the outliers that would change your indexing strategy for both if they need to run and if they could use a little T-SQL help.

Insert / Update / Delete performance: The more indexes you have, the slower your data modifications will be because they have to be written more than once. Wider indexes will be more overhead due to updates hitting it more often, larger index to maintain, and fewer rows per page of data.

Reusability: How many queries can use this index, and how will each of them use it? Is a query using it differently because it’s not referencing the table consistently or because it’s legitimately doing something different? This part is HUGE, and is really going to be a major focus. I didn’t give you a hard time on that query consistency point asking if you cared for no reason!

Memory usage: How much memory is being used, where is it being used, and why? Is that memory being used to fulfill multiple queries (see Reusability, which references Query Consistency, which goes back to Query Tuning)? Could we use less memory if we had a descent index? Is the query that requires all that memory justified and timed properly? These points are starting to mix together and reference themselves, aren’t they? Is indexing strategy an infinite loop?!?!? Yes, yes it is.

Key Lookups: For the queries that use this index, is this a covering index? If not, what would it need to be a covering index? We’ll look at these things: how critical is each query, how often is that query run, how many key lookups does it do, how wide are the total columns we would need to add to be covering, how often are each of those columns updated, what other queries would take advantage of having those columns in there, and is there any filtering being done on those columns?

Maintenance: It’s easy to see having fewer, more narrow indexes would make index rebuilds, index reorgs, and database backups quicker and easier. How about key column order and compression?

TDE: What’s this feature doing in an indexing article?

SQL Edition: Index compression is going to be the biggest one for us today. Online rebuilds can make a big difference, too, but it rarely makes a big difference in what indexes you want.

Pick a table, any table

We’re not going to change the entire database at once.  Partially because it’s overwhelming to you, but mostly because it’s lower risk that’s easier to troubleshoot and roll back if needed. So we’ll pick a single table that we want to have run more efficiently, make a change or two to it, then do it again with either the same table or a different one.

I’m not consistent on how I pick tables.  Although I usually pick one of the ones that’s the largest in the buffer pool that I haven’t made as efficient as I could already, which you can find using the query in  my Cleaning Up the Buffer Pool post.  However, that’s not always how I pick a table.  I’ll also start this off with a query that I wish was running faster, run it with SET STATISTICS IO, TIME ON to see what tables are getting hit in the slowest part, and work on a table that’s getting hit inefficiently here.  There’s no magic to it, just pick a table and reassure the other tables that they’ll get their turn later.

It looks like an infinite loop at first glance because I’ll keep picking tables and loop back to revisit table later, but it’s better to think of it as an upward spiral. That first trip around will give you all kinds of awesome, the second trip will add on to that, and each trip around yields less of an improvement. You could stop wherever you wanted if it wasn’t addictive.

Also as the size of your data changes, the queries hitting your database change, and more, it’s unreasonable to expect your indexing strategy to stay 100% the same.  This isn’t a job where you can ever say you’re really done, just in a better place than you were last week.

I have a table, now what?

At the times of day you want your database to perform great, what’s happening with your table? This may be anything that ever runs on the database for some places, and it may be anything that runs between 8 AM and 5 PM for others.

I’m being as all inclusive as possible by looking at everything that touches the table, so this won’t be as quick and easy as you’d think. Yes, my target audience for this post can create an index for a single query in minutes while I typically spend well over an hour on a single table; how fast you make it through this project isn’t my primary concern.

Once you picked a table to work on look in the proc cache to see what references the indexes on the table.  My query to do that in the same Cleaning Up the Buffer Pool post is good for this, but the one in Querying the Plan Cache is better for viewing an entire table at once. This has every cacheable plan that ran since the last restart of services and hasn’t been forced out of memory. Remember this is a really CPU intensive query that will take several minutes to run and needs to run against prod at a time of day you care about to provide what you need. If you have an extremely busy hour or two, run this as soon as things start to calm down.

Note, there were three different ways something could have avoided you seeing it in the proc cache, and that wasn’t counting if you turned on the typically recommended setting “Optimize for ad-hoc workload” that Kimberly Tripp (b|t) wrote about where you can miss the first run of ad-hoc queries in favor of keeping your memory cleaner. It’s also possible that a query is getting a different plan each time it gets compiled due to stats changing or parameter sniffing, but that affects us a little less since we’re going to make index changes that will change those plans anyways.

The proc cache query is also only capturing index usage. It does capture RID lookups, but not table scans against a heap. You’ll have to look at the modified scripts I put in Querying the Plan Cache to see the table scans because they’re stored differently in the XML.

For now, let’s focus on how things can sneak past our cache and how we can find them.

1 & 2: Was not run since the last restart of services or was forced out of memory. It can be in cache, it’s just not there right now. For that, we’re going to check back on the cache multiple times, and we’re also going to make one or two index changes at a time which will again have us checking back multiple times.

3: Uncacheable plans can happen for several reasons, with the most popular in my experience being temp tables where data was loaded into it then an index was created on the temp table. Whatever the reason, start up extended events or a trace and watch for sql_statement_recompile to help hunt them down. Take this list and search for references of your table to know which ones are relevant. To get bonus points (I’m not keeping score), find out why it’s not getting along with your cache and see if it’s something that should be fixed.

To make things a little more difficult in that step, you’ll also have to look for references to views and functions that reference the table. The views and functions will show up in my Proc Contains Text query, and you’ll have to iterate through that process again.

Keep in mind, this will never be perfect and 100% all-inclusive. I know I said that before, but I need some of the important details repeated to me before they sink in and I have to assume there are others like me. It will be very rare for this to pick up an ad-hoc query that runs for year-end processing. You can use your imagination to find 10 other ways you’ll miss something and still be shocked when a new way pops up.

However, we have enough to move forward, and we’re going to accept the rest as acceptable risk. If you don’t accept some risk you’ll never accept any rewards, it’s just a matter of reducing the risk and knowing enough to make an intelligent decision.

Now that you know what’s running, how is each one referencing the table? Looking at the proc cache, you’ll see predicates and seek predicates, which you’ll combine on a list. You’re going to have to run the stuff that didn’t make it into the proc cache manually on a test server and add them to the list as well.

This is completely overwhelming to do all of it.  The more you do, the more accurate your results will be, but it’s not actually reasonable.  Do what’s reasonable, understand that you’re trading off some level of accuracy for time, and also understand that if you don’t make that tradeoff then you’ll never have time for anything else…not even going home at night.

Here’s what the list could use:

  • Proc or name of SQL Batch
  • How important is it
  • How often does it run
  • When does it run
  • Predicates and Seek Predicates (let’s just call them predicates)
  • Equality columns
  • Range columns
  • Inequality columns
  • Column’s returned
  • Rows returned

If there was a RID or Key Lookup on a reference to a nonclustered index, add the output columns and predicate (not the seek predicate for this case only) from the lookup on here as well.  The seek predicate on a lookup is just the clustered index key or RID bookmark added as hidden key columns on your nonclustered index; they will not help you here.

Now look at this list and look for consistencies. What equality predicates are used a lot? You should be able to find different groups of equality predicates that can accommodate most of your queries, and those are going to be the key columns you’ll consider for your indexes. The first key column is going to be the column all of the queries you want to use this index have in common as an equality column, then iterate through them as the columns are used less and less.

This is not the traditional “order of cardinality” advice I’m sure you’ve heard when creating an index for a specific query, but we’re designing an index for your database, not your query. I’ll go one step further and say if it’s a toss-up between what’s the first key column, consider making it one that’s added sequentially such as DateAdded or ID on tables that see more updates because that will reduce page splits and fragmentation.

A query can take advantage of the chain of key columns starting with the first one. The chain can continue after each equality use. An inequality or range can take advantage of a key column as well, but the first one of these is the end of your chain. Once the chain is broken, everything else can be useful, but only as unordered values that don’t matter if they’re key columns or included columns.

You can stop putting in key columns when either queries stop being able to take advantage of them being ordered or the values you’re getting are either unique or close enough. These key columns aren’t free as Paul Randal (b|t) points out in his post On index key size, index depth, and performance.  If a key column is not very useful, then it’s very useful not to have it there.

I should note that if you’re using an index to enforce uniqueness then it will use all the key columns and none of the included columns to do so.  Based on the last paragraph you don’t want any key columns after it’s unique anyways, so don’t even consider that.  However, included columns aren’t used to calculate uniqueness, so you can make this a covering index if it helps you without hurting the unique constraint.

This process, like any other indexing process, isn’t going to be perfect. You’ll have to weigh your decisions with queries that are more critical or are called more often carry more weight in your decision.

Now that you have your key columns figured out, look at the queries that use more than just those columns. When they reference this index how many rows are they going to return where they have to get more information from the table itself through a lookup? How wide are those columns, and how many other queries are going to do the same? This is the balancing act between adding included columns and accepting key lookups. A key lookup is going to be a nested loop operation making separate calls to get the missing columns from the clustered index (or heap, for those who wish to anger me), so 10,000 key lookups is 10,000 separate calls in a loop. If you have to add a large number of columns to eliminate 10 key lookups then it’s almost never worth it. If you have to add one small column to eliminate 1,000,000 key lookups then it’s almost always worth it. Notice I didn’t use determinate language on those…you’ll have to find your balance, test it, and balance it again.

Some things like adding a column to avoid key lookups may make more of a difference to you than the user, but that doesn’t make it less important. For instance, I just said adding a small column to eliminate 1,000,000 key lookups is almost always worth it. If those 1,000,000 key lookups were from a single execution of a query then the user would probably notice, they might even buy you lunch if you’re lucky. If it was a single key lookup on a query run 1,000,000 times that day it’s still a drop in CPU utilization and a potential 1,000,000 pages from the clustered index that didn’t have to be read into cache. You’re doing this because it adds up to a better running server, not always because it’s noticed.

Your goal here is making an index as reusable as is reasonable. Here are the reasons you’re doing that:

  • Every index will fight to be in cache (assuming you don’t have vastly more memory than databases to fill it), an index that is reusable by many queries will be more likely to already be in cache and that space in cache will be more versatile.
  • Every index is another write process in an Insert, Update, and Delete, you’re trying to cut down on those.
  • Every index needs to be maintained, you’re cutting down on that, too.
  • Every index adds disk space, backup size, backup duration, restore durations, etc..
  • If you use TDE, every time a page is read from disk into memory it is decrypted. A reusable index tends to stay in memory more, reducing the number of times the CPU has to decrypt it. See, I TOLD you this belonged in an indexing strategy post!

Nothing’s free, so here’s what you’re giving up:

  • The index isn’t going to be the prefect index for most queries. Test the performance of your critical queries, but also keep in mind that these indexes are more likely to be in cache which could eliminate physical reads from the execution of those queries.
  • These indexes will tend to be wider than the query needs, which is basically restating that this isn’t going to be the perfect, most efficient index for a query. It also means that physical reads on these indexes will tend to be more expensive as there are fewer rows per page.  Again, keep in mind they’re more likely to be in memory because you’re going with fewer indexes that are shared by more queries.

Once you decide on an index or two to add, there are a couple things to consider.

  • What indexes don’t you want anymore? If a query could use another index slightly more efficiently, it will.  However, if it’s close enough then you want to get rid of that other index because of all those benefits of reusability I just mentioned (weren’t you paying attention?). It’s not a question of if a query would use the other index, it’s a question of if you want it to use it.
  • Some queries “should” use this index based on the key columns, but instead of it showing up as a seek predicate it shows up as a predicate. In these cases either your chain of key columns was broken (if column 2 wasn’t an equality column, column 3 will not be a seek predicate) or this column is not being referenced in a SARGable way.
  • Test in non-prod, not prod. Then test it again.
  • Know you’re accepting risk and understand everything involved the best you can. Have a healthy fear of that risk while also understanding that you took a risk just driving to work this morning.

Once these changes go through keep an eye on how they’re being used over the next couple weeks. If you’re in a rush to make a big impact, start a second table as the first change or two are in progress on the first table. Just don’t get too many changes in motion for a single table at once as that’s typically adding more risk and hiding which changes had positive and negative impacts. This is a process, and the longer it takes you do go through it the better the chance is that you’re doing it right.

If I’m doing this process for someone else who wants consistent improvement without taking on too much time or risk at once, then I like to get on their servers once or twice a month, find one or two changes to suggest, then have those go through testing and implementation.  Afterwards review the results and come up with the next suggestion.  It’s hard to be that patient as a full-time employee, but try.

Cluster It

All of that was talking about nonclustered indexes, but you get to pick a clustered index for your table as well.  Keep in mind this is a bigger change and involves more risk, but it’s also a bigger reward.

So, what do I care about that’s special for a clustered index?

  • Uniqueness
  • Key width
  • Width of columns being queried
  • Column types being returned (some can’t be in nonclustered indexes)
  • Number of rows being returned

The size of your key columns on your clustered index is the MINIMUM size of the key columns on a nonunique nonclustered index, and it’s also the MINIMUM width of the page level of any nonclustered index.  You need to keep that in mind.

However, just because your table has an identity column in it doesn’t mean that’s the best clustered index.  What is the best clustered index is going to vary wildly from table to table; there’s not always going to be a clear answer, and the answer will partially depend on how the table is queried.  I get into that a lot more in my last post, Picking a Clustered Index.  Yes, I wrote that post specifically to keep this one shorter…with mixed results.

If a table is often queried by a relatively small column that’s not unique, but the queries tend to pull back most of the columns in the table and a large number of rows then it’s worth considering using this as part of the clustered index key.

If you don’t then you’re faced with two solutions; you can make a really wide nonclustered index to cover these queries, or you can let the queries decide if they want to do a ton of key lookups or just scan the clustered index.  These don’t sound like fun to me.

You still have to worry about the integrity of your data, so if you’re dropping the unique clustered index with a single column to do this then you almost definitely want to add a unique nonclustered index with that single key column to maintain your data integrity.

Compress It

Index compression is an Enterprise-ONLY feature.

Compression is a very big point to hit on here, even if I’m only giving you the compressed version.  It makes your data smaller on disk (less I/O), smaller in memory (less need for I/O), and actually tends to lower your CPU usage instead of raising it.  I get into a lot more detail in my Data Compression post because I didn’t want to have too much space dedicated to a feature not everyone can use here.

Don’t Forget the Outliers

Go back to that list you made of all the queries hitting a specific table. Were some of the queries different than the rest? There are usually a couple, and that doesn’t necessarily mean there’s an issue. However, I look at these to determine if they are using the table properly.

Are they are joining on all the fields they should be. Sometimes you can get the correct results by joining on 3 of the 4 fields you technically should, so why join on the 4th? Well, index reusability is one of those reasons, because it may not be able to use the proper index because someone skipped a column that happens to be the first key field of the perfect index for this query.

Is the query SARGable? Sometimes you’re joining or filtering on the right fields, but something is written in a way that SQL couldn’t do a direct comparison.

Are you returning too many columns? I’ve seen queries returning 20 columns (or using *, which is a move obvious version of the same thing) to populate a screen that uses 3 of them, and on the SQL side you have a DBA trying to figure out if they should add included columns to an index to make that run more efficiently. The most efficient for this and many other examples is refactoring, not reindexing.

Remember, your goal is to make your server run more efficiently, and tweaking indexes is simply one of your tools. While you’re going through this process keep your eyes open towards how other tools can be used.  SSMS is never going to come up with a warning telling you that you should read a book or two by Itzik Ben-Gan (b|t) or Kalen Delaney (b|t), but I would welcome that change.

Does this negate my previous advice?

If you follow my blog at all, which is suggested in my very biased opinion, you may have seen me talk about Unused and Duplicate Indexes, but I make no mention of them here. Did I forget about them?

No, I did not. This is designing every index you want to have on your table in a reusable way. If that index was not on the list then you’ll want to consider getting rid of it. It’s two ways of looking at the same thing. A complete understanding of both of these methods will help you make intelligent indexing decisions and go as far as you need to for the situation you’re in.

Talk to me

This isn’t a short or easy process, and perhaps I could have worded some of it better.  I enjoy what I do, both writing this post and playing with indexes, and having someone think enough of me to ask me questions on this makes it all the more enjoyable.

I may be rewriting parts of this post as I find ways to reword concepts better, especially as I finalize and tweak my presentation with the same name for which this post is my guide. That presentation will make its debut at SQL Saturday Cleveland on February 6, 2016.

If you feel you can help me improve, please don’t hold back.  I’d rather feel that I’m improving than falsely believe I’m infallible.

Picking a Clustered Index

A Clustered Index is not another term for Primary Key, and more thought should be put into the key columns of the index than always allowing them to default to the PK.

First of all, the primary key is the main way you uniquely identify a row in a table enforcing data integrity, while the clustered index is the order in which a table is stored and retrieved. Although they are typically the same column(s), they are completely separate concepts. The are the same so often that this should be your default, but they are different often enough that you should remember that’s only a default and not a rule.

Example

Let’s have an exception in mind as we go through the details.

  • We have an Orders table which has OrderID as the PK, and 11 other columns including OrderDate, CustomerID, and Status.
  • 90% of our querys are filtered by OrderID; these queries are typically already narrowed down to a single record, but sometimes we have a small handful of OrderID’s in a table where we’re getting more than one a time.
  • When we query by a CustomerID we could be getting up to 5% of the table, and only getting four columns (CustomerID, OrderID, OrderDate, Status)
  • Throughout the day, several people run queries to see what orders were placed for a day, week, or month. These queries vary in other columns they are filtered by and how many columns they return, and several return pratically every column in the table.It seems very obvious to pick OrderID as the clustered index. It’s an identity column, it’s added sequentially, it’s the Primary Key, and 90% of our queries are filtered by it. What more could we hope for, right?

It’s not what I’d pick though

OrderID is the Primary Key, and we’re not going to even consider changing that. However, did you notice that our queries that can filter by OrderID are only pulling back a couple records each, typically only one record? That means the performance of those queries wouldn’t be noticeably hurt if they had to do some key lookups to get the rest of the information. While it’s true that a key lookup will add about 4 reads per row (assuming index depth of 4), a handful of rows means that will add up to 20 or 40 reads total. That’s not an issue.

CustomerID may seem like a logical choice. We could be pulling back 5% of the table for a single customer, and that’s a lot. However, the screen in our app and standard reports only require 4 columns, so we’ll just make a covering nonclustered index on this. Since it’s consistently only using 1/3 of your columns, it’ll be quicker as a nonclustered index anyways.

OrderDate is a little different. It’s rather common to pull back a lot of records, and it’s not too rare to ask for a lot of columns when we do that. This means we have four choices.

      1. Narrow Nonclustered Index – Do key lookups to get other columns. They’re only 4 reads each, but that’s 200,000 reads if they query returns 50,000 rows.
      2. Covering Nonclustered Index – It would have to include pratically every column to avoid key lookups.
      3. Clustered Index (or Table) Scan – Just let it read the entire table. It may be cheaper to read it this way even if you have a nonclustered index because those key lookups add up when you get too many rows.
      4. Make this the first key field of the clustered index – Quick seek with all your columns. There are other things to keep in mind, but this is looking good.

Cluster Date

Ok, so we want to look into making OrderDate our clustered index. It’s not a unique column, so if this was our only key field it would not be a unique index. That’s something you want to avoid because all indexes are unique, it’s just a matter of if you make it that way or if SQL Server has to do it behind your back.

Here’s a rabbit hole that Lisa Jackson-Neblett (b) started me on a couple years ago while I was attending one of David Pless’s (b|t) classes.

A nonunique clustered index will get a new, hidden 4-byte column called the uniquifier that you can’t do anything useful with.  Its value is a 0 when there are no duplicates, then 1 for the first duplicate, and so on.  Ken Simmons (b|t) gets into the details with his post Understanding and Examining the Uniquifier in SQL Server.  There, with that column added on, now it’s unique!

A nonunique nonclustered index will use the clustered index’s key fields to make its key unique.  See this in action in Kendra Little’s (b|t) blog post How to Find ‘Secret Columns’ in Nonclustered Indexes.  That means if you made OrderDate the only key field on your nonunique clustered index, then making a nonunique nonclustered index with the only explicit key field CustomerID would have three key fields, in this order: CustomerID, OrderDate, Uniquifier.

A unique nonclustered index like we’re talking about making on OrderID to enforce the primary key would still need the clustered index’s key, just not as key fields.  Instead it would add OrderDate and Uniquifier as included columns.

Make it Unique

While there are times having a nonunique clustered index is a good idea, this isn’t one of them.  In fact, almost any time you think it’s a good idea it’s because you’re missing something.

In this case it’d be easy to add a second key column of OrderID and call it unique, eliminating the 4-byte uniquifier by adding in what’s probably a 4-byte integer field making the clustered index key the same size.  Even if it was a bigger company that needed a big int for the column, at least you have a useful column.

The nonclustered index on CustomerID will now have the key columns CustomerID, OrderDate, and OrderID.  That’s not that big of a deal because you were planning on including those two columns anyways.  You’ll still declare the index with OrderID, OrderDate, and Status as included columns.  SQL Server will promote OrderID and OrderDate to key columns, but you want them on your definition so they’re still in the index if someone changes the clustered index down the road.  The net effect is that this nonclustered index just got 4-bytes smaller for included columns because you made the clustered index unique.

The nonclustered index on OrderID will still have just the one key field because it’s unique, and it will include OrderDate automatically.  Just like the index on CustomerID, the included columns are 4-bytes smaller now.

Query It

Now that you have these indexes, when you query by OrderID you have to do key lookups on the few rows you return.  It’s more expensive than it was before, but the time this adds is usually not an issue because you’re talking about so few extra reads.

Since you’re sorted by OrderDate and it’s typical to want to know about your recent orders more than historical orders, you’re also being very efficient with your memory.  It’s an advanced piece to look at, but worth mentioning.  The first 10,000,000 leaf-level pages of the clustered index are historical data that is rarely read (probably not in cache), but the last 100,000 pages are more current and read a lot (in cache).  Not only that, but each page has many orders that we need to have in cache, and that difference adds up.  Yes, it’s true we would have had the same effect if we left OrderID as the clustered index, but it’s good to know we didn’t hurt anything.

Fragmented Cluster

With a clustered index as an incremental identity column it’s always adding data to the end of the table, so page splits are rare.  Yes, it’s true that adding another page at the end of the table counts as a page split, but it’s not the kind that slows us down.  Changing the clustered index like this adds this as a concern, making it more likely that this index will need to be cleaned up by our index maintenance task.

We made the first key field OrderDate.  While it’s not guaranteed to be the last row in the table, I’d expect that to be somewhat normal which would avoid fragmentation.  If, however, you have a single order in there with an OrderDate of 2099-01-01, you’re doomed.  Well, not doomed, but every new page added will split the last page 50/50 and write to the second page.  Not only did it have to do more work to split the pages instead of just creating a new one, but it also left a 50% full page while having you start out on a page half way to requiring another page split.

This isn’t a deal-breaker, but it’s another cost we have to keep in mind.

What was that again?

So the primary key wasn’t the best clustered index in this case because of how the table is queried.  By the numbers it’s queried more by the primary key, but we had to look at what the queries were doing as well.  Nothing beats knowing your data.

Although I just talked you through this without running the scripts and testing every piece, it does not mean you can make a change like this without testing (I’m not allowed to, I wrote it into my mental contract with myself).  Get a typical load and run it in non-prod, make this change in non-prod, then run it in non-prod again.  Feel free to measure twice and cut once, I heard that’s a good idea.

They Can’t All Be Clustered

You only have one clustered index on your table because, well, it IS your table.  However, you can have lots of nonclustered indexes.  I will say that if you can tell me how many nonclustered indexes you can have then you’re doing it wrong, but you’re probably doing it wrong if you don’t have any, too.  Look into my next post, Indexing Strategy, to start to get an idea of what you want to do with your nonclustered indexes.

Digging Deeper

The more you learn about indexing the more intelligent your decisions are going to be, so keep learning.  Know the details of what’s in an index, why it’s there, and how it affects you.  Practically everything you do with SQL Server is either modifying or retrieving data from an index.

Here are a couple things to think about.

The clustered index is the table itself ordered by the key columns, and you can pretty much think of it like a nonclustered index that automatically includes every other column in the table.  Although you can have a table without a clustered index, it’s typically a heap of trouble you don’t want.

Because all of the columns in the clustered index are at least in the leaf-level pages of all nonclustered indexes, any update to any of the key columns of the clustered index will be an update to all of the nonclustered indexes.  I’ve seen times where this was an issue, I’ve seen times where it wasn’t an issue, and I’ve seen many more times where the clustered index key just isn’t on columns that get updated.  In this case there may be rare times when an order date is changed, but nothing to worry about.  Besides, most indexes on this table would want the OrderDate column in there anyways.

I may have left you with the impression that data is physically stored in the order of the key field, but that’s not how SQL Server does it.  Read Gail Shaw’s (b|t) post Of clustered indexes and ordering to see SQL Server is really doing.  Basically, SQL Server knows which order to retrieve pages, although they’re not stored in that order because fragmentation happens.  Also, the rows on each page aren’t actually stored in order, either.

A couple days after I originally released this post Matan Yungman (b|t) released When Should You Use Identity as a Clustered Index Key?, which is viewing this from the inserts side.  It’s different from my discussion, but still the Clustered Index.  View things from as many sides as possible if you want the best answer possible…read his post!

I’m sure there’s more to say about the key columns of a clustered index I’m not thinking about right now.  Let me know in the comments below and I’ll add to this list as needed.

Data Compression

Data compression is often misunderstood to cost CPU in exchange for smaller size on disk.  Somewhat true, but that simple explanation ignores other savings that often result in net drop in CPU utilization.

Full disclosure: This is an Enterprise-ONLY feature introduced in SQL 2008.  It is engrained in the structure of your data, so it also means you can’t take a backup of a database that has a compressed index and restore it to anything other than Enterprise or Developer Editions.

Here are the simple facts we’ll play with:

  • Two levels – row-level and page-level
  • Page-level is row-level plus extra compression
  • Compression ratios vary by column types, index width, and data
  • Data is compressed both on disk and in memory
  • CPU goes both ways, and it needs to be tested
    • Uses CPU to compress on writes and index maintenance
    • Uses CPU to decompress when used by a query
    • Saves CPU with fewer physical reads
    • Saves CPU with fewer logical reads
    • And more…

Abstract Thought

This post is at a level of abstraction that doesn’t get into what happens in the background.  My goal is to encourage you to test it out, understand why it helps, and be able to explain that in your change control process.

For those of you who aren’t satisfied with my “Gas pedal make car go fast” explanation, Jes Borland (b|bob|t) wrote A Look Inside SQL Server Row and Page Compression, and Brad McGehee (b|t) wrote A Quick Introduction to Data Compression in SQL Server 2008.

You can even dive into more details such as using different levels of compression on each partition of an index, or even talking to Joey D’Antoni (b|t) about the archival levels of compression on columnstore indexes.

There’s a lot of detail on how compression can cost CPU, but the details that save CPU are typically only mentioned in passing without doing a deep dive into the topic.  Data Compression: Strategy, Capacity Planning and Best Practices mentions that less Logical I/O is less to consume CPU.  SQL Server Database Compression is indirectly mentioning having a smaller B+Tree structure.

The purpose of this post isn’t to earn anyone a doctorate (or claim that I’m at that level), it’s more of a practitioner level.

What’s it do?

Each page is the same 8kb size but contains more data per page, as per Captain Obvious.  This means less space on disk and backups.  Those are nice, but I don’t care too much about that.

Then you read the data into memory so queries can use it.  This is a physical I/O to disk that goes through the CPU (using extra CPU to decrypt it if you use TDE) to make it into memory.  It stays compressed when in memory, so all of your indexes (not just this one) have more room to hang around and avoid more physical I/Os and the costs I just mentioned.

Finally, a query needs to use the data, and that has positives (+) and negatives (-).  The data is more likely to be in cache (+) because it’s smaller and a page with more data is more likely to be referenced. It’s easier to get into cache if it wasn’t there already (+). Then it’s easier to get to the data because the smaller data may have fewer levels in the B+Tree (+). Along the way it has to decompress the root and intermediate level pages (-) which are always row-level compressed when you use any level of compression then decompress the leaf-level pages (-) which are compressed at the level you picked.  However, there are fewer pages, which results in less Logical I/O (+).

You’re not going to accurately figure out the positives and negatives of that last paragraph.  The important part is that you know there are positives AND negatives, which means you put away the calculus and just run some tests.

My experience is that if the data is compressed by 25% or more than it helps more than it hurts.  Find how much you’ll save by running sp_estimate_data_compression_savings for both row-level and page-level compression.  If you don’t get much extra compression with page-level then don’t even test it, it’s an added expense that needs to be justified.

What Compresses Well?

The hard way is to analyze each column, its data type, the data in that column, the width of the index, etc..  You can read the links in the Abstract Thought section to see what Brad and Jes have to say about it if you want.  This will be very important if you’re designing tables and keeping how compressible the data is in mind, but less so if you’re compressing already existing indexes.

The easy way (my personal favorite) is to just run sp_estimate_data_compression_savings I just mentioned and actually compress the indexes on a non-prod server.  Computers are good at math, let them do it.

How to Test

I’m not diving deep into testing here, but there are three things that stand out.

  • How much memory are you saving?
  • How do your queries perform?
  • How much faster is the data pulled from disk?

For how much memory you would save, look at my Cleaning Up the Buffer Pool post to see how much memory that index is using.  Since you’re only changing how much space the data takes and not the columns of the indexes here, you can just multiply that by the new compression ratio.  Use the actual ratio comparing the index size in prod to where you’re testing in non-prod to make sure it’s accurate.  Yes, if you have a 10 GB index which tends to be 100% in cache that you just compressed 80%, it will be like you added 8 GB of memory in many ways.

I do query performance and how much faster the data is pulled from disk together, because that’s how it’s done in the real world.  Pick your queries that hit that index, typically by looking in the plan cache or an XEvent session.  Then, on a non-prod server, run the queries both with and without DBCC DROPCLEANBUFFERS, again, on a non-prod server.

You can remove compression on any index, down to the partition level, by doing ALTER INDEX … REBUILD WITH (DATA_COMPRESSION = NONE).  Adding compression is the same statement with ROW or PAGE instead of NONE.

Sum It Up

Do this all in non-prod.

  1. See what compresses well
  2. Test it
  3. Test it again

The End

Let’s hear from you.  If you needed more data to make an informed choice, throw it in the comments where others can benefit from your experience, and I may even edit the post to add it in.  Also, if this is making a big difference for a lot of people, I’ll do what I can to tweak the SEO and help more people find this through search engines.

The best compliment is a question.  It means you value my expertise enough to want my thoughts and opinions.

Querying the Plan Cache

I love working with indexes, and I need to know what’s using them to work on them intelligently. Most of that information is already there waiting for you to query it. Luckily, Jonathan Kehayias (b|t) did the hard work for us in his post Finding what queries in the plan cache use a specific index, and I could modify his code to get entire tables.

Remember that you’re querying XML and that’s a CPU intensive process. However, you’re also looking for what’s in cache which is most relevant during or just after your busiest time of day on prod. The longer you wait, the more chance a query will be flushed from cache for one reason or another, although waiting a couple hours typically isn’t a problem on a server that’s not under extreme stress.

This first script will find all references to the indexes of a table in the plan cache, including key and RID lookups. However, table scans against heaps are in the XML a little different, and that’s what my second query is for. Hopefully you have a clustered index on every table you want to run this against, but you’ll probably need both of these.

If you’re only looking for a single index in the cache, I already have that query on my Cleaning up the buffer pool post. However, I’m working on some new stuff that’s way too long to put scripts in-line, so I had to create this post to add as a reference. I’ll try to remember to update this post as I have everything done, but keep an eye out for my “Indexing Strategy” post and my presentation by the same name I hope to debut at the Cleveland SQL Saturday in February 2016.

Table Index Usage

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
DECLARE @TableName SYSNAME = '[ShiverMeTuples]'; 
DECLARE @DatabaseName SYSNAME;
 
SELECT @DatabaseName = '[' + DB_NAME() + ']';
 
WITH XMLNAMESPACES
   (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan')
SELECT
    n.value('(@StatementText)[1]', 'VARCHAR(4000)') AS sql_text,
    --n.query('.'),
    cp.plan_handle,
    i.value('(@PhysicalOp)[1]', 'VARCHAR(128)') AS PhysicalOp,
    i.value('(@EstimateRows)[1]', 'VARCHAR(128)') AS EstimateRows,
    i.value('(@EstimateIO)[1]', 'VARCHAR(128)') AS EstimateIO,
    i.value('(@EstimateCPU)[1]', 'VARCHAR(128)') AS EstimateCPU,
	cp.usecounts,
    i.value('(./IndexScan/@Lookup)[1]', 'VARCHAR(128)') AS IsLookup,
    i.value('(./IndexScan/Object/@Database)[1]', 'VARCHAR(128)') AS DatabaseName,
    i.value('(./IndexScan/Object/@Schema)[1]', 'VARCHAR(128)') AS SchemaName,
    i.value('(./IndexScan/Object/@Table)[1]', 'VARCHAR(128)') AS TableName,
    i.value('(./IndexScan/Object/@Index)[1]', 'VARCHAR(128)') as IndexName,
    --i.query('.'),
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./OutputList/ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS output_columns,
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./IndexScan/SeekPredicates/SeekPredicateNew//ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS seek_columns,
    RIGHT(i.value('(./IndexScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'), len(i.value('(./IndexScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)')) - charindex('.', i.value('(./IndexScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'))) as Predicate,
    query_plan
FROM (  SELECT plan_handle, query_plan
        FROM (  SELECT DISTINCT plan_handle
                FROM sys.dm_exec_query_stats WITH(NOLOCK)) AS qs
        OUTER APPLY sys.dm_exec_query_plan(qs.plan_handle) tp
      ) as tab (plan_handle, query_plan)
INNER JOIN sys.dm_exec_cached_plans AS cp 
    ON tab.plan_handle = cp.plan_handle
CROSS APPLY query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/*') AS q(n)
CROSS APPLY n.nodes('.//RelOp[IndexScan/Object[@Table=sql:variable("@TableName") and @Database=sql:variable("@DatabaseName")]]' ) as s(i)
--WHERE i.value('(./IndexScan/@Lookup)[1]', 'VARCHAR(128)') = 1
ORDER BY 3 
OPTION(RECOMPILE, MAXDOP 4);

Table (Heap) Scans

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
DECLARE @TableName SYSNAME = '[ItsAHeapOfSomething]'; 
DECLARE @DatabaseName SYSNAME;
 
SELECT @DatabaseName = '[' + DB_NAME() + ']';
 
WITH XMLNAMESPACES
   (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan')
SELECT
    n.value('(@StatementText)[1]', 'VARCHAR(4000)') AS sql_text,
    n.query('.'),
    cp.plan_handle,
    i.value('(@PhysicalOp)[1]', 'VARCHAR(128)') AS PhysicalOp,
    i.value('(./TableScan/@Lookup)[1]', 'VARCHAR(128)') AS IsLookup,
    i.value('(./TableScan/Object/@Database)[1]', 'VARCHAR(128)') AS DatabaseName,
    i.value('(./TableScan/Object/@Schema)[1]', 'VARCHAR(128)') AS SchemaName,
    i.value('(./TableScan/Object/@Table)[1]', 'VARCHAR(128)') AS TableName,
    --i.value('(./TableScan/Object/@Table)[1]', 'VARCHAR(128)') as TableName,
    i.query('.'),
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./OutputList/ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS output_columns,
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./TableScan/SeekPredicates/SeekPredicateNew//ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS seek_columns,
    RIGHT(i.value('(./TableScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'), len(i.value('(./TableScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)')) - charIndex('.', i.value('(./TableScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'))) as Predicate,
	cp.usecounts,
    query_plan
FROM (  SELECT plan_handle, query_plan
        FROM (  SELECT DISTINCT plan_handle
                FROM sys.dm_exec_query_stats WITH(NOLOCK)) AS qs
        OUTER APPLY sys.dm_exec_query_plan(qs.plan_handle) tp
      ) as tab (plan_handle, query_plan)
INNER JOIN sys.dm_exec_cached_plans AS cp 
    ON tab.plan_handle = cp.plan_handle
CROSS APPLY query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/*') AS q(n)
CROSS APPLY n.nodes('.//RelOp[TableScan/Object[@Table=sql:variable("@TableName") and @Database=sql:variable("@DatabaseName")]]' ) as s(i)
--WHERE i.value('(./TableScan/@Lookup)[1]', 'VARCHAR(128)') = 1
OPTION(RECOMPILE, MAXDOP 4);

SQL Server Error Log Reader

Reading the SQL Server Error Log is miserable.  It contains very useful information you should address as soon as possible, or at least know that it’s happening.  However, it’s hidden between so many informational messages that it’s hard to find, then it’s spread out between multiple files for every server reboot or automated file rollover event you may have set up.

Many DBAs skim these files, but when there’s a single login failure mixed into log backups running every 5 minutes for 100 databases then they’re just happy to have found something.  That login failure tells you nothing, just that someone should have been more careful typing in their password, right?  When you’re just happy you were even able to find something then you’re almost certainly not going to see it clearly enough to notice a trend, such as that login failure happens every Sunday between 10:00 PM and 10:15 PM.  However, if you knew that then you could tell someone that there’s an automated job that’s failing, it’s obviously part of a bigger process because the time varies a little, but it’s consistent enough to say it’s definitely a process.

So, the trick is to get past the junk and to the useful information.  You can listen to Warner Chaves (b|t) in his Most Important Trace Flags post and turn on trace flag 3226 to stop backup information from going to the logs, but I’m always paranoid (it’s part of the job) that it just may come in useful some day.  I know it never has, but I leave it in there anyways.

Even if you do take out information from the logs like that, it’s still a flat file that’s difficult to analyze for any number of reasons.  We’re just a T-SQL kind of group, and flat files just fall flat.

As with everything in SQL Server, I cheat my way through it.  This time I pull it into a temp table, delete the stuff I’m ignoring (please be very, very careful about what you ignore because you’ll never know it happened), then look at the results.  If there’s a login failure then I’ll uncomment the section that deletes everything except a single error and trends will pop out at me.  If I wanted to do more advanced analysis I would run queries doing aggregates of any kind against the temp table that was created.  Everything’s in the format you’re used to analyzing, so you can do crazy things without going crazy.

DECLARE @dStart DateTime 
	, @dEnd DateTime
	, @MaxLogFiles Int 

SELECT @dStart = GetDate()-30
	, @dEnd = GetDate()-0
	, @MaxLogFiles = 5

--Pulls into #TempLog because an empty log file causes errors in the temp table
--If there are no records, don't pass the issues onto your #Log table and return the results

IF OBJECT_ID('tempdb..#Log') IS NOT NULL BEGIN
	DROP TABLE #Log
END

IF OBJECT_ID('tempdb..#TempLog') IS NOT NULL BEGIN
	DROP TABLE #TempLog
END

CREATE TABLE #Log
(
	LogDate DateTime
	, ProcessInfo NVarChar(50)
	, LogText NVarChar(1000)
)

CREATE TABLE #TempLog
(
	LogDate DateTime
	, ProcessInfo NVarChar(50)
	, LogText NVarChar(1000)
)

DECLARE @Num int
SELECT @Num = 0

WHILE @Num < @MaxLogFiles BEGIN
	TRUNCATE TABLE #TempLog 

	INSERT INTO #TempLog 
	exec xp_readerrorlog @Num, 1, null, null, @dStart, @dEnd

	IF @@ROWCOUNT > 0 BEGIN
		INSERT INTO #Log 
		SELECT *
		FROM #TempLog
	END ELSE BEGIN
		SELECT @Num = @MaxLogFiles
	END
		
	SELECT @Num = @Num + 1
END 

/*
--Uncomment to trend out a specific message and ignore the rest
DELETE #Log
WHERE LogText NOT LIKE 'Login failed for user ''WhatAreYouDoingToMe?!?!?''%'
--*/

--Ignore most of the log file rollover process
--Keep "Attempting to cycle" and "The error log has been reinitialized" if you want to confirm it happened and succeeded
DELETE #Log 
WHERE LogText LIKE '%(c) Microsoft Corporation%'
	OR LogText LIKE 'Logging SQL Server messages in file %'
	OR LogText LIKE 'Authentication mode is MIXED.'
	OR LogText LIKE 'System Manufacturer: %'
	OR LogText LIKE 'Server process ID %'
	OR LogText LIKE 'All rights reserved.'
	OR LogText LIKE 'Default collation: %'
	OR LogText LIKE 'The service account is %'
	OR LogText LIKE 'UTC adjustment: %'
	OR LogText LIKE '(c) 2005 Microsoft Corporation.'--Should I be ignoring this or fixing it?
	OR LogText LIKE 'Microsoft SQL Server % on Windows NT %'
	OR LogText LIKE 'The error log has been reinitialized. See the previous log for older entries.'
	OR LogText LIKE 'Attempting to cycle error log.%'

--Ignore databases being backed up and integrity checks running, assuming you verify this some other way.
--I don't want to complain to try to have these removed because I may need that info someday; today isn't that day.
DELETE #Log
WHERE LogText LIKE 'Log was backed up%'
	OR LogText LIKE 'Database differential changes were backed up%'
	OR LogText LIKE 'Backup database with differential successfully %'
	OR LogText LIKE 'Backup database successfully %'
	OR LogText LIKE 'Database backed up%'
	OR LogText LIKE 'DBCC CHECK% found 0 errors %'
	OR LogText LIKE 'CHECKDB for database % finished without errors %'

--We all have vendor databases...
--Ignore the stuff where it keeps making sure the setting is where the setting was.
DELETE #Log
WHERE LogText LIKE 'Configuration option % changed from 30 to 30.%'
	OR LogText LIKE 'Configuration option % changed from 5 to 5.%'
	OR LogText LIKE 'Setting database option COMPATIBILITY_LEVEL to 100 for database ReportServer%'
	OR LogText LIKE 'Configuration option ''user options'' changed from 0 to 0. Run the RECONFIGURE statement to install.'

--Now your own custom ones
--Just be careful.  You'll rarely read logs without this script once you see how easy it is.
--If you put it on the ignore list, you won't see it again.
--I have starting and stopping traces on mine, because my monitoring software likes to start and stop them a lot
----I'm accepting the risk that I won't see other people starting and stopping traces.
DELETE #Log
WHERE LogText LIKE 'Know what risk you''re taking on by putting stuff in here'
	OR LogText LIKE 'You will rarely read logs without this, so you won''t see these ever again'
	OR LogText LIKE 'DBCC TRACEON 3004,%'
	OR LogText LIKE 'DBCC TRACEON 3014,%'
	OR LogText LIKE 'DBCC TRACEON 3604,%'
	OR LogText LIKE 'DBCC TRACEOFF 3604,%'
	OR LogText LIKE 'DBCC TRACEON 3605,%'
	OR LogText LIKE 'Error: %, Severity:%'--They give the english version next
	OR LogText LIKE 'SQL Trace ID % was started by %'
	OR LogText LIKE 'SQL Trace stopped.%'
	OR LogText LIKE 'Changing the status to % for full-text catalog %'
	OR LogText LIKE 'I/O was resumed on database %'
	OR LogText LIKE 'I/O is frozen on database %' 

/*
--When mirroring gives me trouble it lets me know by flooding the logs
--I uncomment this to see if there were other issues in the middle of all that.
DELETE #Log 
WHERE LogText LIKE 'Database mirroring is inactive for database%'
	OR LogText LIKE 'The mirroring connection to%has timed out%'
	OR LogText LIKE 'Database mirroring is active with database%'
--*/

/*
--This is only useful if you're using the trace flag 1222
--Only show the line that says 'deadlock-list'.  Remove this if you need to see the deadlock details.
--Note, only use this when needed.  It will give you a 1 second blind spot for every deadlock found.
--Why aren't you using extended events anyways?
DELETE L
FROM #Log L
	INNER JOIN #Log L2 ON L.LogDate BETWEEN L2.LogDate AND DateAdd(second, 1, L2.LogDate) AND L.ProcessInfo = L2.ProcessInfo 
WHERE L2.LogText = 'deadlock-list'
	AND L.LogText <> 'deadlock-list'
--*/

SELECT * 
FROM #Log
ORDER BY LogDate DESC

Don’t Ignore Me

Anything you ignore you won’t see here again. It’s still in the logs, but not in what you’re reading on your screen when you mentally check the logs off as being read through.  If you’re ignoring anything make sure it either doesn’t matter or you’re watching for it another way.

Backups are the first thing to be ignored.  Yes, yes, they ran successfully, they do that a lot, don’t tell me about them.  That can be good advice gone horribly wrong.  Do you have another way of saying I absolutely know I have backups taken of everything?

DBCC CheckDB ran successfully is next on the list.  Same thing goes for it, except more DBAs miss verifying that this is running and also miss running it.  If you ignore it, how are you verifying that it ran?

I don’t care how you do it.  Do what works best for you, just do something.

Be Careful

I’ll just end by saying be careful again. This code is a life saver when it’s not shooting you in the foot.

Give More Feedback

I write on my blog and get a couple comments at best. I talk at conferences and a large part of the audience fills out evals at the time.  Then I often wonder if I’m making a difference while rarely, if ever, knowing if anyone actually used what they learned at work.  There are some confidence issues at hand here, which I was convinced were just limited to me.  However, talking to a couple others in the SQL community, it’s not just me.

In fact, I told one of the leading respondents to SQL Server Central forums questions that he really made a difference in my career with the quantity and quality of the knowledge he shares.  His response was a lot like I’d imagine mine to be.  A humbleness and sincere thank you with a follow up that it’s very difficult to know if the work you put into the community really makes a difference.  That’s saying something considering that he’s in a more personal position than me with his online presence because he’s often answering user questions while I offer unfocused unsolicited advice.

The blog posts, articles, and forum help you see online is all done on a volunteer basis.  Sure, some people get paid $25 for an article, which is a lower hourly rate than an entry-level DBA if you think about how much work they put into them.  Some people write blog posts to promote their business or help their careers while knowing without a doubt that well over 99% of people who read what they have to say will never hire them.  Yes, there are ways you can say it’s not on a volunteer basis, but if any of us were really in it for the money then almost all of us would opt for something more lucrative such as setting up a lemonade stand on the corner with our kids.

That wasn’t even getting into in-person events like SQL Saturdays where volunteers put everything together then ask for volunteers to come speak, many of whom pay their own travel expenses.  Full disclosure, it’s not completely unpaid, we get invited to a free dinner the night before and typically get a free shirt.  Both of these are amazing benefits because I’m eating dinner with people I look up to then I have a shirt that I wear to work every time I need a good confidence boost, so I can’t say it’s really free.  By-the-way, every SQL Saturday is numbered, and I they all have an email address of SQLSaturday###@SQLSaturday.com.  Help keep the motivation going and force me to update that masked email address to have four digits!

So, you may notice that I write at length about not getting much feedback on my only post that does not allow comments.  I even went out of my way and deleted the name of the guy whose answers I liked so much on SQL Server Central.  Why?!?!  It’s not as counter-productive as it seems.  I know this post didn’t make your career better, I know it’s not likely to change your life, and this isn’t a plea for you to comment on THIS post or for people who inspired ME.

This post is a challenge to you.  Think back to blog posts that helped make you awesome at work.  Think about conferences that were put together so well that all you noticed were the great learning opportunities.  Then go out and comment as publically as possible how it helped, adding as many details as you can.  After that, keep this in mind when you’re learning in the future, be it free or paid events, reading blog posts, or using #sqlhelp on twitter.  Say it all, constructive criticism, compliments, offers to buy someone a beer, don’t hold back!  If you want more of something, speak up.  We have an amazing community, and I want more of it!

By the way, the guy from SQL Server Central likes plain old Budweiser.

Extended Events Intro

Extended Events is supposed to be taking over for most of Profiler and server-side tracing functionality, but there were people like me out there that took their time making the switch. For me the excuse was SQL Batch Completed isn’t in 2008 / 2008 R2, most databases are slow to move off of 2008 because it’s hard to say 2008 can’t do everything you want, and, well, I really like SQL Batch Completed!!!

Now I’m losing my excuses. It’s been three years since 2012 came out and all the new servers I’m installing are 2012 or 2014, so I lost that excuse. Then I came up with “I don’t know how to use them, but I know how to use something that does almost the same thing”; that’s great logic during an outage, but terrible logic for the long term. So, I’m learning it, still learning it, and I have something to help out those who made it to 2012 but are still stuck on the “I don’t know how to use them” phase of denial.

In several of my blog posts I referenced my favorite server-side trace. Capture everything on the server that takes over X seconds, with 5 being my favorite starting place. Yes, this adds about 1ms to anything that takes over 5 seconds, which adds up to…insignificant. Seriously, if this kills your server then it was dead already, but you will see arguments against creating this for long-term runs.  I don’t agree with those arguments, but they’re your servers and I want you to make an informed decision on what you run.

Anyways, here’s how I set it up with Extended Events, for which I used Jonathan Kehayias’s (b|t) Trace to XE Converter to get started.

IF EXISTS (SELECT 1 FROM sys.server_event_sessions WHERE name = 'DurationOver5Sec')
	DROP EVENT SESSION [DurationOver5Sec] ON SERVER;
GO
CREATE EVENT SESSION [DurationOver5Sec]
ON SERVER
ADD EVENT sqlserver.rpc_completed(
	ACTION 
	(
		  sqlserver.client_app_name	-- ApplicationName from SQLTrace
		, sqlserver.client_hostname	-- HostName from SQLTrace
		, sqlserver.client_pid	-- ClientProcessID from SQLTrace
		, sqlserver.database_id	-- DatabaseID from SQLTrace
		, sqlserver.request_id	-- RequestID from SQLTrace
		, sqlserver.server_principal_name	-- LoginName from SQLTrace
		, sqlserver.session_id	-- SPID from SQLTrace
	)
	WHERE 
	(
			duration >= 5000000
	)
),
ADD EVENT sqlserver.sql_batch_completed(
	ACTION 
	(
		  sqlserver.client_app_name	-- ApplicationName from SQLTrace
		, sqlserver.client_hostname	-- HostName from SQLTrace
		, sqlserver.client_pid	-- ClientProcessID from SQLTrace
		, sqlserver.database_id	-- DatabaseID from SQLTrace
		, sqlserver.request_id	-- RequestID from SQLTrace
		, sqlserver.server_principal_name	-- LoginName from SQLTrace
		, sqlserver.session_id	-- SPID from SQLTrace
	)
	WHERE 
	(
		duration >= 5000000
	)
),
ADD EVENT sqlos.wait_info(
	ACTION 
	(
		  sqlserver.client_app_name	-- ApplicationName from SQLTrace
		, sqlserver.client_hostname	-- HostName from SQLTrace
		, sqlserver.client_pid	-- ClientProcessID from SQLTrace
		, sqlserver.database_id	-- DatabaseID from SQLTrace
		, sqlserver.request_id	-- RequestID from SQLTrace
		, sqlserver.server_principal_name	-- LoginName from SQLTrace
		, sqlserver.session_id	-- SPID from SQLTrace
	)
	WHERE
	(
		duration > 5000 --This one is in milliseconds, and I'm not happy about that
            AND ((wait_type > 0 AND wait_type < 22) -- LCK_ waits
                    OR (wait_type > 31 AND wait_type < 38) -- LATCH_ waits
                    OR (wait_type > 47 AND wait_type < 54) -- PAGELATCH_ waits
                    OR (wait_type > 63 AND wait_type < 70) -- PAGEIOLATCH_ waits
                    OR (wait_type > 96 AND wait_type < 100) -- IO (Disk/Network) waits
                    OR (wait_type = 107) -- RESOURCE_SEMAPHORE waits
                    OR (wait_type = 113) -- SOS_WORKER waits
                    OR (wait_type = 120) -- SOS_SCHEDULER_YIELD waits
                    OR (wait_type = 178) -- WRITELOG waits
                    OR (wait_type > 174 AND wait_type < 177) -- FCB_REPLICA_ waits
                    OR (wait_type = 186) -- CMEMTHREAD waits
                    OR (wait_type = 187) -- CXPACKET waits
                    OR (wait_type = 207) -- TRACEWRITE waits
                    OR (wait_type = 269) -- RESOURCE_SEMAPHORE_MUTEX waits
                    OR (wait_type = 283) -- RESOURCE_SEMAPHORE_QUERY_COMPILE waits
                    OR (wait_type = 284) -- RESOURCE_SEMAPHORE_SMALL_QUERY waits
	--OR (wait_type = 195) -- WAITFOR
                )
	)
)
ADD TARGET package0.event_file
(
	SET filename = 'DurationOver5Sec.xel',
		max_file_size = 10,
		max_rollover_files = 5
)
WITH 
(
	MAX_MEMORY = 10MB
	, MAX_EVENT_SIZE = 10MB
	, STARTUP_STATE = ON
	, MAX_DISPATCH_LATENCY = 5 SECONDS
	, EVENT_RETENTION_MODE = ALLOW_MULTIPLE_EVENT_LOSS
);

ALTER EVENT SESSION DurationOver5Sec
ON SERVER
STATE = START;
	

What’s it all mean?

This captures all SQL Batch Completed and RPC Completed events that took over 5 seconds along with any waits that took over 5 seconds. Seems kind simple and much easier to read than the script to create a trace, but there are some things to point out here.

  • Duration – Milliseconds or Microseconds
  • File Location
  • Restart on server reboot

First, duration, is it milliseconds or microseconds? Trick question, it’s both!!! Ola Hallengren (b|t) opened This Connect Item which resulted in Microsoft apologizing and saying they’ll fix it in SQL 2016. They can “fix” it one of two ways. Either they’ll document them as being different scales and your script will still work without any modifications, or they’ll make them all be the same scale and the script I have here will behave differently on different versions of SQL Server. Anyways, the script I have here is 5 seconds all around, which is 5,000,000 microseconds for the SQL and 5,000 milliseconds for the waits.

Continuing on the duration confusion, the wait info’s duration still isn’t what you might think it is.  This is the duration for a single instance of a wait, not cumulative waits for the duration of a specific query or batch.  If you made a loop that executed 10 times calling a wait for delay of 1 second each time then those are 10 separate 1 second waits.  More important, if your query is doing physical reads from disk then it’s a ton of small PageIOLatch waits, not one large one this session will see.  I understand why it has to be this way, but understanding and enjoying are two different things.

The rest of this isn’t nearly as confusing, I promise.

The file location I’m using is just a filename without a path, which will default to where your error log files are. It’s going to be a much more complex name once SQL Server is done with it, and not just a underscore with a rollover count like server-side traces.  However, it will start how it’s written here and you can use wildcards to say which files you want to read.

Now when you restarted the server, or just SQL services, with server-side traces you’d have to manually start that trace again if you wanted it to run all the time. This is exactly how the script I have here works, too. However, Extended Events also added “Startup State” which means it will start when the server starts. The only issue I have with this is that it will ONLY automatically start when the services start. That’s all fine and good if you’re a patient person….it’s not all fine and good for me, so I manually start it when it’s created.

The other minor details I can see having questions on are on the Max Dispatch Latency and Event Retention Mode. These are limits on how it writes to the file location I’m using. Max Dispatch means that SQL Server will write it to the output file within that many seconds after the event actually happens, so it’s obviously an asynchronous action. Event Retention Mode can give SQL Server permission to say it’s too overworked to write the entries and skip them if needed, and I chose to tell SQL Server to drop as many as it wants to if I’d be slowing it down.  It’s all documented in BOL, and it’d be good for you to read through that instead of just listening to me picking out select details.

So, how do you read this? It’s going to put it all in XML, isn’t it!?!?

If you can read this, Kendra Little (b|t) may suggest you’re a demon robot! (While her comment makes me laugh, that post is a great XE reference or it wouldn’t be on here.) Well, she’s probably right in calling this next script names because friendly robots don’t treat your CPU like this. In fact, if you’re on a stressed server or want to do a lot of analysis on this, it’s going to be best to copy the .xel files to a server you care a little less about and shred the XML there.

Unfortunately, I wrote my script to automatically find the files and add a wildcard character, so you’d have to modify my script to run it on another server. The point is getting you started with as little frustration as possible, and this is perfect for that reason. The biggest catch is that the session has to be running for this to work without changes because it’s grabbing the full file name with path from the DMVs for running sessions.

Anyways, here goes….

DECLARE 
	@SessionName SysName 
	, @TopCount Int = 1000

--SELECT @SessionName = 'UserErrors'
SELECT @SessionName = 'DurationOver5Sec'
--SELECT @SessionName = 'system_health'
/* 
SELECT * FROM sys.traces

SELECT  Session_Name = s.name, s.blocked_event_fire_time, s.dropped_buffer_count, s.dropped_event_count, s.pending_buffers
FROM sys.dm_xe_session_targets t
	INNER JOIN sys.dm_xe_sessions s ON s.address = t.event_session_address
WHERE target_name = 'event_file'
--*/

SET STATISTICS IO, TIME ON

IF OBJECT_ID('tempdb..#Events') IS NOT NULL BEGIN
	DROP TABLE #Events
END

IF OBJECT_ID('tempdb..#Queries') IS NOT NULL BEGIN
	DROP TABLE #Queries 
END

DECLARE @Target_File NVarChar(1000)
	, @Target_Dir NVarChar(1000)
	, @Target_File_WildCard NVarChar(1000)

SELECT @Target_File = CAST(t.target_data as XML).value('EventFileTarget[1]/File[1]/@name', 'NVARCHAR(256)')
FROM sys.dm_xe_session_targets t
	INNER JOIN sys.dm_xe_sessions s ON s.address = t.event_session_address
WHERE s.name = @SessionName
	AND t.target_name = 'event_file'

SELECT @Target_Dir = LEFT(@Target_File, Len(@Target_File) - CHARINDEX('\', REVERSE(@Target_File))) 

SELECT @Target_File_WildCard = @Target_Dir + '\'  + @SessionName + '_*.xel'

--SELECT @Target_File_WildCard

SELECT TOP (@TopCount) CAST(event_data AS XML) AS event_data_XML
INTO #Events
FROM sys.fn_xe_file_target_read_file(@Target_File_WildCard, null, null, null) AS F
ORDER BY File_name DESC
	, file_offset DESC 

SELECT  EventType = event_data_XML.value('(event/@name)[1]', 'varchar(50)')
	, Duration_sec = CAST(event_data_XML.value ('(/event/data[@name=''duration'']/value)[1]', 'BIGINT')/CASE WHEN event_data_XML.value('(event/@name)[1]', 'varchar(50)') LIKE 'wait%' THEN 1000.0 ELSE 1000000.0 END as DEC(20,3)) 
	, CPU_sec = CAST(event_data_XML.value ('(/event/data[@name=''cpu_time'']/value)[1]', 'BIGINT')/1000000.0 as DEC(20,3))
	, physical_reads_k = CAST(event_data_XML.value ('(/event/data  [@name=''physical_reads'']/value)[1]', 'BIGINT')/1000.0 as DEC(20,3))
	, logical_reads_k = CAST(event_data_XML.value ('(/event/data  [@name=''logical_reads'']/value)[1]', 'BIGINT') /1000.0 as DEC(20,3))
	, writes_k = CAST(event_data_XML.value ('(/event/data  [@name=''writes'']/value)[1]', 'BIGINT')/1000.0 as DEC(20,3))
	, row_count = event_data_XML.value ('(/event/data  [@name=''row_count'']/value)[1]', 'BIGINT')
	, Statement_Text = ISNULL(event_data_XML.value ('(/event/data  [@name=''statement'']/value)[1]', 'NVARCHAR(4000)'), event_data_XML.value ('(/event/data  [@name=''batch_text''     ]/value)[1]', 'NVARCHAR(4000)')) 
	, TimeStamp = DateAdd(Hour, DateDiff(Hour, GetUTCDate(), GetDate()) , CAST(event_data_XML.value('(event/@timestamp)[1]', 'varchar(50)') as DateTime2))
	, SPID = event_data_XML.value ('(/event/action  [@name=''session_id'']/value)[1]', 'BIGINT')
	, Username = event_data_XML.value ('(/event/action  [@name=''server_principal_name'']/value)[1]', 'NVARCHAR(256)')
	, Database_Name = DB_Name(event_data_XML.value ('(/event/action  [@name=''database_id'']/value)[1]', 'BIGINT'))
	, client_app_name = event_data_XML.value ('(/event/action  [@name=''client_app_name'']/value)[1]', 'NVARCHAR(256)')
	, client_hostname = event_data_XML.value ('(/event/action  [@name=''client_hostname'']/value)[1]', 'NVARCHAR(256)')
	, result = ISNULL(event_data_XML.value('(/event/data  [@name=''result'']/text)[1]', 'NVARCHAR(256)'),event_data_XML.value('(/event/data  [@name=''message'']/value)[1]', 'NVARCHAR(256)'))
	, Error = event_data_XML.value ('(/event/data  [@name=''error_number'']/value)[1]', 'BIGINT')
	, Severity = event_data_XML.value ('(/event/data  [@name=''severity'']/value)[1]', 'BIGINT')
	, EventDetails = event_data_XML 
INTO #Queries
FROM #Events

SELECT q.EventType
	, q.Duration_sec
	, q.CPU_sec
	, q.physical_reads_k
	, q.logical_reads_k
	, q.writes_k
	, q.row_count
	, q.Statement_Text
	, q.TimeStamp
	, q.SPID
	, q.Username
	, q.Database_Name
	, client_app_name = CASE LEFT(q.client_app_name, 29)
					WHEN 'SQLAgent - TSQL JobStep (Job '
						THEN 'SQLAgent Job: ' + (SELECT name FROM msdb..sysjobs sj WHERE substring(q.client_app_name,32,32)=(substring(sys.fn_varbintohexstr(sj.job_id),3,100))) + ' - ' + SUBSTRING(q.client_app_name, 67, len(q.client_app_name)-67)
					ELSE q.client_app_name
					END  
	, q.client_hostname
	, q.result
	, q.Error
	, q.Severity
	, q.EventDetails
FROM #Queries q
--WHERE eventtype NOT IN /*rather typical filtering*/ ('security_error_ring_buffer_recorded', 'sp_server_diagnostics_component_result', 'scheduler_monitor_system_health_ring_buffer_record')
	--AND eventtype NOT IN /*specific troubleshooting filtering*/ ('connectivity_ring_buffer_recorded', 'wait_info')
ORDER BY TimeStamp DESC 

The top is all I typically edit. The Session Name is ‘DurationOver5Sec’ for this purpose, but as you can see it can also read some of the information from the system_health session. The TopCount is just saying I’m only going to look at the most recent 1,000 records BEFORE I shred and filter it, mostly because I love my CPU and this changes how much XML I’m throwing at it.

No matter how I write this to filter before playing with XML or try to be 100% safe, my scripts will not be perfect for you.  In fact they may crash your server. Unfortunately, I’m only half kidding, because nothing is 100%. The space is limited to 50 MB, the overhead of writing to this file should be fine since we’re only working with events that took over 5 seconds asynchronously with data loss allowed, so I don’t foresee any issues in most cases unless you change these numbers. However, reading them gets expensive and you have to use the most caution here.  Keep in mind that you can put the .xel files on any server with SQL installed, even developer edition, and query it.

Why’d I write this and what else is going through my head?

The inspiration for this post actually came from a question a former coworker asked me quite a while ago, just seeing if I knew a lot about Extended Events.  At the time the technically correct answer was “I don’t know”, but instead I told him that although I didn’t have experience with Extended Events that he could look at Jonathan Kehayias’s (b|t) blog, specifically his XEvent A Day Series.  However, I also warned him that Jonathan’s work tends to go deeper than a beginner is prepared for and that he should start with his coworker, Erin Stellato’s (b|t), version that is easier to start with, specifically her Making the Leap From Profilers to Extended Events session she made available on YouTube. This actually inspired two blog posts, I just haven’t written the How to Say “I don’t know” Better post yet.

More Links

It is so wrong that I originally posted this without linking to Erin Stellato’s Extended Events Stairway Series on SQL Server Central.  There, problem is fixed.

Have fun, experiment (with queries), and ask questions.