Should You Use Index Hints?

Published on: 2018-07-31

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One of the things that the SQL Server query optimizer does is determine how to retrieve the data requested by your query.

Usually it does a pretty good job, which is a great because if it didn’t then we’d be spending most of our days programming sorting and joining algorithms instead of having fun actually working with our data.

Sometimes the query optimizer has a lapse in judgement and createds a less-than-efficient plan, requiring us to step in and save the day.

Index Hints Give You Control

One way to “fix” a poor performing plan is to use an index hint.  While we normally have no control over how SQL Server retrieves the data we requested, an index hint forces the  query optimizer to use the index specified in the hint to retrieve the data (hence, it’s really more of a “command” than a “hint”).

Sometimes when I feel like I’m losing control I like using an index hint to show SQL Server who’s boss.  I occasionally will also use index hints when debugging poor performing queries because it allows me to confirm whether using an alternate index would improve performance without having to overhaul my code or change any other settings.

…But Sometimes That’s Too Much Power

While I like using index hints for short-term debugging scenarios, that’s about the only time they should be used because they can create some pretty undesirable outcomes.

For example, let’s say I have this nice simple query and index here:

CREATE INDEX IX_OwnerUserId_CreationDate_Includes
ON dbo.Posts (OwnerUserId, CreationDate) INCLUDE (AcceptedAnswerId, ClosedDate, CommentCount, FavoriteCount, LastActivityDate);

SELECT
    OwnerUserId,
    AcceptedAnswerId
FROM
    dbo.Posts
WHERE
    OwnerUserId < 1000

This index was specifically created for a different query running on the Posts table, but it will also get used by the simple query above.

Executing this query without any hints causes SQL Server to use it anyway (since it’s a pretty good index for the query), and we get decent performance: only 1002 logical reads.

I wish all of my execution plans were this simple.

Let’s pretend we don’t trust the SQL Server optimizer to always choose this index, so instead we force it to use it by adding a hint:

SELECT
    OwnerUserId,
    AcceptedAnswerId
FROM
    dbo.Posts WITH (INDEX(IX_OwnerUserId_CreationDate_Includes))
WHERE
    OwnerUserId < 1000

With this hint, the index will perform exactly the same: 1002 logical reads, a good index seek, etc…

But what happens if in the future a better index gets added to the table?

CREATE INDEX IX_OwnerUserId_AcceptedAnswerId_Includes
ON dbo.Posts (OwnerUserId, AcceptedAnswerId) INCLUDE (LastEditorUserId, ParentId);

If we run the query WITHOUT the index hint, we’ll see that SQL Server actually chooses this new index because it’s smaller and we can get the data we need in only 522 logical reads:

This execution plan looks the same, but you’ll notice the smaller, more data dense index is being used.

If we had let SQL Server do it’s job, it would have given us a great performing query!  Instead, we decided to intervene and hint (ie. force) it to use a sub-optimal index.

Things Can Get Worse

The above example is pretty benign – sure, without the hint SQL Server would have read about half as many pages, but this isn’t a drastic difference in this scenario.

What could be disastrous is if because of the hint, the query optimizer decides to make a totally different plan that isn’t nearly as efficient.  Or if one day someone drops the hinted index, causing the query with the hint to down right fail:

Index hints  can be nice to use in the short-term for investigating, testing, and debugging.  However, they are almost never the correct long-term solution for fixing query performance.

Instead, it’s better to look for the root-cause of a poor performing query: maybe you need to rebuild stats on an index or determine if the cardinality estimator being used is not ideal.  You might also benefit from rewriting a terribly written query.

Any of these options will likely help you create a better, long-term, flexible solutions rather than forcing SQL Server to use the same hard-coded, potentially sub-optimal index forever.

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Pinal Dave Helps Me Fix My Performance Tuning Problems

Published on: 2018-07-24

Be sure to check out this week’s video on YouTube.

This week I was fortunate enough to film a video in collaboration with Pinal Dave, the SQL Authority himself.  Pinal is creative, hilarious, and kind; making this video with him was A BLAST!

Although the video is a little tongue in cheek, Pinal’s recommendations are very real: I’ve encountered plenty of scenarios where these solutions fixed slow queries.  Will these recommendations fix the problem in every situation?  Of course not, but they are a great place to start.

Instead of creating a text version of the concepts covered in the video (you should really watch it), I thought it would be fun to do a behind-the-scenes narrative of how the video came together because it is unlike any other project I’ve done before.

The Idea

After agreeing to make a video together, we tossed around a few ideas.  Because we live in different time zones, we thought it would be a fun to do something where I kept waking Pinal up in the middle of the night.

We iterated over what SQL Server examples to use (originally the second example was going to show my queries running out of space because autogrowth being turned off).  We also ended up adding another example after my wife suggested that having it build to three scenarios instead of two would be funnier – I agree!

Asynchronous Filming

You’ve probably already figured it out, but I didn’t really wake Pinal up in the video (honestly, I think midnight would be too early to wake him up anyway; in our back and forth emails, I was seeing responses from him that were in the 1-2am range).

I filmed a preliminary version of my parts of the video, very roughly edited them together, and sent it over to Pinal.

He then filmed his segments, giving me lots of great footage (I’m not sure if it was ad-libbed or not, but I was dying of laughter when watching through his clips).

Then I re-filmed my parts to try to match his dialog as closely as possible.  Re-filming my parts also allowed me to self-edit and not ramble as much.

Everything Else

After that, it was just the usual process of editing, color correction, audio processing, etc…

I’m happy with how it turned out, especially given all of the technical challenges we had with filming separately.

Major thanks again to Pinal for being supportive and willing to make a fun SQL Server video.  Enjoy!

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Why Is My VARCHAR(MAX) Variable Getting Truncated?

Published on: 2018-05-15

Watch this week’s episode on YouTube.

Sometimes SQL Server doesn’t do what you tell it to do.

Normally that’s ok – SQL is a declarative language after all, so we’re supposed to tell it what we want it to do, not how we want it done.

And while that’s fine for most querying needs, it can become really frustrating when SQL Server decides to completely disregard what you explicitly asked it to do.

Why Is My VARCHAR(MAX) Truncated to 8000 Characters?

A prime example of this is when you declare a variable as VARCHAR(MAX) because you want to assign a long string to it.  Storing values longer than 8000 characters long is the whole point of VARCHAR(MAX), right?

DECLARE @dynamicQuery VARCHAR(MAX);

SET @dynamicQuery = REPLICATE('a',8000) + 'b'

SELECT @dynamicQuery as dynamicQueryValue, LEN(@dynamicQuery) AS dynamicQueryLength

If we look at the above query, I would expect my variable @dynamicQuery to be 8001 characters long; it should be 8000 letter ‘a’s followed by a single letter ‘b’.  8001 characters total, stored in a VARCHAR(MAX) defined variable.

But does SQL Server actually store all 8001 characters like we explicitly asked it to?

No:

First we can see that the LEN() of our variable is only 8000 – not 8001 – characters long!

Copying and pasting our resulting value into a new query window also shows us that there is no character ‘b’ at position 8001 like we expected.

The Miserly SQL Server

The reason this happens is that SQL Server doesn’t want to store something as VARCHAR(MAX) if none of the variable’s components are defined as VARCHAR(MAX).  I guess it doesn’t want to store something in a less efficient way if there’s no need for it.

However, this logic is flawed since we clearly DO want to store more than 8000 characters.  So what can we do?

Make Something VARCHAR(MAX)

Seriously, that’s it.  You can do something like CAST the single character ‘b’ as VARCHAR(MAX) and your @dynamicQuery variable will now contain 8001 characters:

But casting a single character as VARCHAR(MAX) isn’t very intuitive.

Instead, I recommend casting a blank as VARCHAR(MAX) and prefixing it to the start of your variable string.  Leave yourself a comment for the future and hopefully you’ll remember why this superfluous looking piece of code is needed:

-- using CAST('') to force SQL to define
-- as varchar(MAX)
SET @dynamicQuery =  CAST('' AS varchar(MAX))
	+ REPLICATE('a',8000)+ 'b'

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