sp_BlitzIndex: Added warning for persisted sample rates.

[ReeceGoding]

Closes #3679 .

This was disturbingly easy. You will want to check over it very carefully. The only place where I really had to think was the dynamic SQL. I don't have super old versions of SQL Server hanging around, so I have not been able to test any branches other than the one that my 2022 Docker container hits.

The URL that I have given is the best that I could find. I cannot think of anyone who has spoken with the voice of experience about PERSIST_SAMPLE_PERCENT.

I might make an issue for us also not having incremental statistics, but the merge conflict will be a pain unless this is merged first.

[BrentOzar]

This needs to handle Azure SQL DB and a default sampling rate as shown in the comments for the pull request.

I'm not saying you have to code it @ReeceGoding - I can code this later - but just making a note and moving on during a live stream.

[ReeceGoding]

Will do. Thanks @BrentOzar!

I enjoyed the stream. Clicking on the screenshot would have saved you some time!

[ReeceGoding]

Wow. Did not expect to find this edge case. This is valid syntax

CREATE STATISTICS OH_WOW
ON [StackOverflow2010].[dbo].[Posts]
(Score)
WITH SAMPLE 123 ROWS,
PERSIST_SAMPLE_PERCENT = ON;

So we get this...

even though you are not allowed to use a decimal for anything like WITH SAMPLE 12.3456 PERCENT. For example, this is not valid syntax

CREATE STATISTICS OH_NO
ON [StackOverflow2010].[dbo].[Posts]
(Score)
WITH SAMPLE 12.34 PERCENT,
PERSIST_SAMPLE_PERCENT = ON;

Maybe I'll have to make @UsualStatisticsSamplingPercent a float.

[ReeceGoding]

I believe that what I've just pushed does all that @BrentOzar asks. See the commit message for details.

Here's a test script for Brent.

/* On a fresh StackOverflow 2010 copy, there is nothing new here. */
USE [StackOverflow2010];
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0;
EXEC sp_BlitzIndex @SkipStatistics = 1, @Mode = 0;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4;
EXEC sp_BlitzIndex @SkipStatistics = 1, @Mode = 4;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4, @UsualStatisticsSamplingPercent  = 20;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4, @UsualStatisticsSamplingPercent  = 26.5555555;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4, @UsualStatisticsSamplingPercent  = NULL;

/* Now let's make some stats */
CREATE STATISTICS TotallyDefault
ON [StackOverflow2010].[dbo].[Posts]
(Score);

CREATE STATISTICS HundredPercentNotPersisted
ON [StackOverflow2010].[dbo].[Posts]
(Body)
WITH SAMPLE 100 PERCENT;

CREATE STATISTICS FullScanNotPersisted
ON [StackOverflow2010].[dbo].[Posts]
(AnswerCount)
WITH FULLSCAN;

CREATE STATISTICS TwentyFourPercentNotPersisted
ON [StackOverflow2010].[dbo].[Posts]
(OwnerUserId)
WITH SAMPLE 24 PERCENT;

CREATE STATISTICS TwentySixPointFivePercentInRowsNotPersisted
ON [StackOverflow2010].[dbo].[Posts]
(PostTypeId)
WITH SAMPLE 990308 ROWS;

CREATE STATISTICS FullScanPersisted
ON [StackOverflow2010].[dbo].[Posts]
(AcceptedAnswerId)
WITH FULLSCAN, PERSIST_SAMPLE_PERCENT = ON;

CREATE STATISTICS HundredPercentPersisted
ON [StackOverflow2010].[dbo].[Posts]
(ClosedDate)
WITH SAMPLE 100 PERCENT, PERSIST_SAMPLE_PERCENT = ON;

CREATE STATISTICS TwentyFourPercentPersisted
ON [StackOverflow2010].[dbo].[Posts]
(CommentCount)
WITH SAMPLE 24 PERCENT, PERSIST_SAMPLE_PERCENT = ON;

CREATE STATISTICS TwentyFivePointSixPercentInRowsPersisted
ON [StackOverflow2010].[dbo].[Posts]
(Title)
WITH SAMPLE 990308 ROWS, PERSIST_SAMPLE_PERCENT = ON;

/*
    Now try the old calls again
*/
/*
4 rows.
1 Brent's.
2 of mine report that [StackOverflow2010].[dbo].[Posts].[System Or User Statistic].[TwentyFourPercentPersisted].[OwnerUserId]
and [StackOverflow2010].[dbo].[Posts].[System Or User Statistic].[TwentyFivePointSixPercentInRowsPersisted].[Title]
have unexpected sample rates.
This is good, since we set them to stupid values.
The other 1 of mine reports that 2 statistics have 100% persisted but do not say which, but we did one with 100% and another with fullscan so all is as expected.
*/
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0;
/* As above. */
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4;
/* Reports nothing from the new stuff. */
EXEC sp_BlitzIndex @SkipStatistics = 1, @Mode = 0;
/* Reports nothing from the new stuff. */
EXEC sp_BlitzIndex @SkipStatistics = 1, @Mode = 4;
/*
5 rows.
1 Brent's.
2 of mine have been seen before.
The 2 others are from [StackOverflow2010].[dbo].[Posts].[System Or User Statistic].[FullScanPersisted].[AcceptedAnswerId]
and [StackOverflow2010].[dbo].[Posts].[System Or User Statistic].[HundredPercentPersisted].[ClosedDate]
*/
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4, @UsualStatisticsSamplingPercent  = 20;
/*
5 rows.
1 Brent's.
3 of mine have been seen before.
1 is new, reporting that one statistic having a persisted sample rate matching the 26.5556% I asked for has been found.
Those 990308 rows that I asked for must have been a good idea on my machine.
*/
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4, @UsualStatisticsSamplingPercent  = 26.5555555;
/*
5 rows.
1 Brent's.
All seen before.
*/
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 4, @UsualStatisticsSamplingPercent  = NULL;

/* All three of these are identical to the previous three. */
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0, @UsualStatisticsSamplingPercent  = 20;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0, @UsualStatisticsSamplingPercent  = 26.5555555;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0, @UsualStatisticsSamplingPercent  = NULL;

/*
    None of these error. Handling this, if at all, is a decision I'll leave to Brent.
    The 0 case is interesting because sys.dm_db_stats_properties can report persisted_sample_percent as zero
    and we could also entirely legitimately persist a sample percentage so close to zero that I cannot distinguish it from comparisons to zero-the-float being imprecise.
*/
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0, @UsualStatisticsSamplingPercent  = 600;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0, @UsualStatisticsSamplingPercent  = 0;
EXEC sp_BlitzIndex @SkipStatistics = 0, @Mode = 0, @UsualStatisticsSamplingPercent  = -200;

[BrentOzar]

This looks great! Thanks for turning that around quickly while I'm still aware of what the issues were, hahaha. I'm going to merge this as-is because if there's anything wrong, it's just going to be small stuff and I can tweak stuff in the dev branch, but this looks like it's fine as-is. Thanks Reece!