Another neat little feature in SQL Server 2022 is Query Store Hints. This is the ability to apply a query hint through Query Store rather than having to modify existing code or fiddle around with plan guides.

Query hints are a way to influence optimizer behaviour towards generating desired execution plans for a given query. The word “hint” is a bit of a misnomer as usually they mandate what you wish to happen.

I’m not a big fan of leaping straight away to use query hints as they force the behaviour rather than letting the optimizer work out the best plan for your query. This means you may implement hints that you see as desirable for the current issue you are trying to fix, but they create problems for different executions of the query, or if the data profile changes. In general it’s better to see if you can rewrite the SQL code to achieve what you want, but sometimes that’s not possible and a hint ends up being the best way to go – or at least a good short term solution.

The great thing about applying hints through query store is that it can be done without altering application code. That means the DBA can apply a temporary hint while waiting for code to be fixed – or in extremis use them against third party applications where there is not the ability to get code changed.

I first read about them in Bob Ward’s book SQL Server 2022 Revealed. You can also read about them in Grant Fritchey’s excellent (and long!) book SQL Server 2022 Query Performance Tuning.

Applying a hint is simple as long as the query has a plan stored in query store. First you need to obtain the query_id for the query. One method for doing that is to use the following query I got from this stackexchange post:

SELECT
qsq.query_id,
qsq.last_execution_time,
qsqt.query_sql_text
FROM sys.query_store_query qsq
INNER JOIN sys.query_store_query_text qsqt
ON qsq.query_text_id = qsqt.query_text_id
WHERE
qsqt.query_sql_text LIKE '%your query text%';

Armed with the query_id, you can then apply the hint using the sys.sp_query_store_set_hints stored procedure. For example, if my query_id was 100 and I want to apply the RECOMPILE hint then I could do that with this SQL:

EXEC sys.sp_query_store_set_hints 100, N'OPTION(RECOMPILE)';

You can also view which queries have hints applied using the sys.query_store_hints view.

Finally if you want to clear a hint you have previously applied (such as the one above), you can do that as follows:

EXEC sp_query_store_clear_hints @query_id = 100;

Query store does not support all query hints. You see the full list of those supported (and not) at:

https://learn.microsoft.com/en-us/sql/relational-databases/system-stored-procedures/sys-sp-query-store-set-hints-transact-sql?view=sql-server-ver16

And that’s it – it’s that simple. Thanks for reading!

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