What does Azure SQL DB Automatic Index Tuning actually do, and when?

 4 years ago
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Azure SQL DB’s Automatic Tuning will create and drop indexes based on your workloads. It’s easy to enable – just go into your database in the Azure portal, Automatic Tuning, and then turn “on” for create and drop index:



Let’s track what it does, and when. I set up Kendra Little ‘s DDL trigger to log index changes , which produces a nice table showing who changed what indexes, when, and how:


Contents of Kendra’s logging table (click to zoom)

I wanted to do that because the documentation is a little hand-wavy about when these changes actually take effect:

You can either manually apply tuning recommendations using the portal or you can let Automatic tuning autonomously apply tuning recommendations for you. The benefits of letting the system autonomously apply tuning recommendations for you is that it automatically validates there exists a positive gain to the workload performance, and if there is no significant performance improvement detected, it will automatically revert the tuning recommendation. Please note that in case of queries affected by tuning recommendations that are not executed frequently, the validation phase can take up to 72 hrs by design.

To give my Azure SQL DB some horsepower to crank through these truly terrible queries, I gave it 8 cores, then fired up SQLQueryStress to run 20 simultaneous queries against it, and let it run for a day.

Setting up my workload

I took the queries from lab 2 in my Mastering Index Tuning class . We start with the  Stack Overflow database with every table having a clustered index, but no nonclustered indexes at all. Students run a workload with SQLQueryStress, which populates the missing index DMVs and the plan cache, and then you have to put the pieces together to figure out the right indexes. Needless to say, it’s a Mastering class, and it’s purposely designed with pitfalls: if you rely on Clippy’s missing index recommendations, you’re gonna have a bad time.

I didn’t expect Clippy to be a master by any means – this automatic tuning feature is included with your Azure SQL DB bill at no extra charge. It’s free index tuning. I’m pretty much happy with ANY index improvements when they’re free! It’s a great feature for businesses that can’t afford to have a professional performance tuner hand-craft the finest artisanal indexes. (Also, those folks make mistakes.)

After letting the workload run overnight, here’s what Azure SQL DB’s performance metrics looked like:


100% CPU and 100% IO – for those of you who are new to performance tuning, the idea isn’t to score 100 points out of 100. (At the same time, it’s not to score 0 either, because then you’re overspending on resources.) We definitely need some index help – and good news, sp_BlitzIndex reports lots of opportunities for tuning:


Zooming in a little:


Plenty of opportunities on the Posts, Votes, Comments, Badges, and Users tables.

What did Automatic Tuning PLAN to do?

Over in the Azure portal, you can see what recommendations Automatic Tuning came up with:


I love it! All three of those look like good choices. I’m not disappointed that it only picked indexes on 3 of the 5 tables, nor am I disappointed that it picked so few indexes – I would always much rather have it err on the conservative side. That’s great!

That page in the portal doesn’t show the index includes, but if you drill down a level on each index, the includes are shown:


You can even click “View script” to get the creation T-SQL. I do wish they’d name ’em with the fields they’re on, but I understand that might be challenging given that folks may use really long field names. I can live with these names.

ON [dbo].[Badges] ([Name], [Date]) 
CREATE NONCLUSTERED INDEX [nci_wi_Votes_CA77C838AF3A22438365553699CCA079] 
ON [dbo].[Votes] ([PostId], [VoteTypeId], [UserId]) 
INCLUDE ([CreationDate]) WITH (ONLINE = ON)
CREATE NONCLUSTERED INDEX [nci_wi_Users_60AB08E4E3999D39C705FF069F220FFE] 
ON [dbo].[Users] ([Reputation]) 
INCLUDE ([DownVotes], [UpVotes]) WITH (ONLINE = ON)

The Badges and Users indexes exactly matched the missing index DMVs’ recommendations. However, the Votes index was different, and it’s different in a way that gets me all tingly inside. Here were the missing index recommendations on the Votes table:


And here was the recommendation from Azure:

CREATE NONCLUSTERED INDEX [nci_wi_Votes_CA77C838AF3A22438365553699CCA079] 
ON [dbo].[Votes] ([PostId], [VoteTypeId], [UserId]) 
INCLUDE ([CreationDate]) WITH (ONLINE = ON)

Azure managed to merge the first two recommendations together in a way that works successfully! Neither of the DMVs had included all 4 of those fields in a single index, but Azure did. Oh, Clippy, I love you, you’ve gone off to boarding school and passed high school chemistry. (Or maybe this is Clippy’s big sister, and I’m falling in love. Look, let’s not get bogged down in this metaphor.)

What did it ACTUALLY do, and when?

The above screenshots imply that this create-index statement is executing right now. The documentation says “Executing” means “The recommendation is being applied.” but that definitely isn’t true: the Users index shows as “Executing” as of 10:03PM, but hours later, the index still wasn’t there, and no create-index scripts were running.

I guessed maybe Azure was waiting for a lull in the load (love it!) so I stopped the load test and waited. About half an hour later, the index on Users magically appeared. I say “magically” because the DDL trace on index creations didn’t capture what Azure did. That is a bit of a bummer because it’ll make tracking what happened just a little bit harder. We’re going to have to use one mechanism (DDL triggers) to track user-modified indexes, and another mechanism to track Azure-modified indexes.

Azure exposes its tuning actions in sys.dm_db_tuning_recommendations :


Buckle up: the state and details columns are…JSON. Here are the example details for an index. <sigh> Developers love JSON because they can change the contents without having to change the database structure – they can just name a column “details” and then stuff it full of hot garbage without any forethought. I know, it’s not normalized stuff like you’re used to with the DMVs, but stay with me – this will come in handy a few paragraphs from now.

I happened to catch one of the indexes as it was being created. Here’s the live query plan in PasteThePlan :


It’s building the index with the Power of Dynamic SQL™. Here’s the query it’s running – it’s parameterized, but I switched the parameters into a declare, and ran it through format-sql.com to make it easier to read:

DECLARE @sql_exception_autotuning_disabled int,
        @sql_exception_null_table_name int,
        @sql_exception_null_index_name int,
        @schema_name nvarchar(3),
        @table_name nvarchar(5),
        @index_name nvarchar(45),
        @is_online int,
        @is_auto_created int,
        @index_field0 nvarchar(6),
        @index_field1 nvarchar(10),
        @index_field2 nvarchar(6),
        @included_field0 nvarchar(12)
DECLARE @advisor_name NVARCHAR(120) = 'CREATE_INDEX';
DECLARE @exception_msg AS NVARCHAR(MAX);
SELECT @isAutotuningOn = actual_state
FROM sys.database_automatic_tuning_options
WHERE name = @advisor_name;
IF (@isAutotuningOn IS NULL OR @isAutotuningOn = 0)
    SET @exception_msg = 'Autotuning is not enabled for advisor ' + @advisor_name;
    THROW @sql_exception_autotuning_disabled, @exception_msg, 1;
DECLARE @exception_message AS NVARCHAR(MAX);
IF QUOTENAME(@schema_name) IS NULL
    SET @schema_name = 'dbo';
    SET @exception_message = 'table_name can not be null.';
    THROW @sql_exception_null_table_name, @exception_message, 1;
    SET @exception_message = 'index_name can not be null.';
    THROW @sql_exception_null_index_name, @exception_message, 1;
SET @stmt
      + QUOTENAME(@schema_name) + '.' + QUOTENAME(@table_name) + ' (' + QUOTENAME(@index_field0) + ', '
      + QUOTENAME(@index_field1) + ', ' + QUOTENAME(@index_field2) + ') ' + 'INCLUDE (' + QUOTENAME(@included_field0)
      + ')';
DECLARE @stmt_options NVARCHAR(MAX) = '';
IF @is_online = 1
    IF @stmt_options <> ''
        SET @stmt_options = @stmt_options + ', ';
    SET @stmt_options = @stmt_options + 'ONLINE = ON';
IF @is_auto_created = 1
    IF @stmt_options <> ''
        SET @stmt_options = @stmt_options + ', ';
    SET @stmt_options = @stmt_options + 'AUTO_CREATED = ON';
IF @stmt_options <> ''
    SET @stmt = @stmt + ' WITH (' + @stmt_options + ')';
SET @stmt = @stmt + ';';
EXEC (@stmt);

Interesting. That means the script you get in the portal isn’t exactly the same index creation script that Microsoft is actually running – note that this one is terminated with a semicolon, for example, but the portal’s version wasn’t.

How did that affect the server while it ran?

Regular readers may recall our recent adventure, How fast can a $21,468/mo Azure SQL DB load data? In that, I kept bumping up against transaction log throughput limits that seemed strangely low, like USB thumb drive slow.

Well, this is only an 8-core (not 80-core) server, so I did expect index creation (which is a write-heavy activity) to bump up against the log limits hard, and it did. Here’s a screenshot of a 60-second sample of sp_BlitzFirst while the Votes index creation was happening:


A few notes:

  • Log file growing – this strikes me as really odd because I’d loaded this entire database from scratch recently, going from 0 to 300+ GB. The log file shouldn’t have been small, and there hadn’t been write activity in the last half-hour before the index creations. There should have been plenty of space in the log file – which leads me to think that Microsoft is actually – and I hope you’re sitting down for this – shrinking the log file. For shame. That’s a real bummer, causing customer slowdowns for something that we all know is a terrible anti-pattern.
  • CXCONSUMER waits with an average of 467 seconds each? Each?!?
  • INSTANCE_LOG_RATE_GOVERNOR – ah, our throttle friend.
  • Physical write volumes (bottom of the screenshot) – the 8-core instance was able to write about 2GB of data in 60 seconds. Again, terrible USB thumb drive territory, but…not that much worse than the 80-core instance, which really makes me question what’s happening with the scalability of the IO in Azure SQL DB. (I’ve got a blog post coming up on that, and the short story is that while the documentation  says IO scales linearly per-core, it…doesn’t. Not even close.)

After the index creations finished, I fired up my workload again.

How Automatic Tuning validates its changes

After the workload started again, the Azure portal showed the measurements it was taking for each of the indexes:


Badges index savings

This Badges index seems to have paid off well with huge DTU savings, although one of my queries regressed. (I’m curious how they’re measuring that given that many of the queries in my workload are joins across multiple tables, 3 of which got new indexes in this last round of index tuning.)

The Users table didn’t fare quite as well, only seeing minor improvements:


DTU savings on dbo.Users

In a perfect world, I’d love to be able to click on “Queries with regressed performance,” see the queries involved, and figure out how to hand craft better indexes for them. Unfortunately, while you can go to Query Insights, you’re left to start over with the filtering, trying to figure out which queries were involved.

The portal doesn’t do that, but…we might be able to, someday. Remember how I said that the JSON data in sys.dm_db_tuning_recommendations would come in handy? Well, for the automatic-plan-forcing part of Azure’s Automatic Tuning, the improved & regressed query plan IDs are stored in the JSON, and Grant Fritchey wrote a query to fetch ’em . There’s no reason Microsoft couldn’t do that same thing with the improved & regressed queries for the automatic index tuning – after all, they made the Details column a generic JSON dumping ground. They could add the query data in there, and here’s a feedback request to make that happen.

About six hours later, 2 of the 3 indexes showed as validated. Here’s the validation for the Votes index:


Note that 2 queries got better, and 1 query got worse – but I can’t tell what that one query is. Again, no drilldown on “Queries with regressed performance,” and the Query Insights tab just takes you to the database’s overall insights, leaving you to start searching from scratch. Sure would be nice to have this here feature.

Summary: I love it.

Things I love:

  • It’s conservative
  • It’s doing deduplication, merging recommendations together
  • It’s not trying to apply the indexes during peak loads
  • It does a way better job of validating its changes than most DBAs

Things I don’t mind:

  • The execution scheduling is opaque
  • The index names aren’t great
  • The indexes take a long time to apply (but that’s an Azure SQL DB storage limitation, apparently)

Things I don’t like:

  • The validation report doesn’t show me which queries got worse or better (and I know Azure knows)

This feature is really appealing for the classic use case of a database server that doesn’t get any tender loving care from a database administrator. I’m sure the deeper I look at it, the more edge cases I’ll find, but…who cares? I have that same feeling about Clippy’s missing index recommendations, but I still rely on those every day as a starting point when I parachute into a new server. Used wisely, they can help you do a better job of tuning and let you focus more on the hard edge cases.

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