No, Really
When I talk to clients about using dynamic SQL, they’re usually under the misconception that those plans can’t get reused.
That may be true under some circumstances when:
- It’s not properly parameterized
- You use EXEC only and not sp_executesql
Under more favorable circumstances, dynamic SQL gets run, executed, and plans cached and reused with the same frequency as stored procedures.
Now, dynamic SQL isn’t exactly the same as stored procedures. There’s a lot you can do with those that just looks a mess in dynamic SQL, especially longer bits of code.
In today’s post, we’re going to look at how the Parameter Sensitive Plan (PSP) optimization works with dynamic SQL.
Bright, Sunshiny
I just learned how to spell “sunshiny”. Don’t let anyone ever tell you there’s nothing left to learn.
To keep up the sunshiny visage of today’s post, let’s get a TL;DR here: PSP does work with parameterized dynamic SQL.
Here’s an example, using a query with a parameter eligible for the PSP optimization.
DECLARE
@sql nvarchar(MAX) =
N'',
@parameters nvarchar(MAX) =
N'@ParentId int';
SELECT
@sql += N'
SELECT
c = COUNT_BIG(*)
FROM dbo.Posts AS p
WHERE p.ParentId = @ParentId;
';
EXEC sys.sp_executesql
@sql,
@parameters,
0;
EXEC sys.sp_executesql
@sql,
@parameters,
184618;
Both executions here get the option(plan per value... text at the end that indicates PSP kicked in, along with different query plans as expected.
Being Dense
Writing the not-good kind of dynamic SQL, like so:
SELECT
@sql = N'
SELECT
c = COUNT_BIG(*)
FROM dbo.Posts AS p
WHERE p.ParentId = ' + CONVERT(nvarchar(11), 0) + ';';
You will of course get different execution plans, but you’ll get a new execution plan for every different value that gets passed in. You will not get the PSP optimization.
This is not a good example of how you should be writing dynamic SQL. Please don’t do this, unless you have a good reason for it.
Anyway, this is good news, especially for parameterized ORM queries that currently plague many systems in crisis that I get to see every week.
Fun.
Thanks for reading!
Going Further
If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that and need to solve performance problems quickly.
