Performance: CTE in an “IN”-Operator

I’ve got a query with a CTE (getting all children of a parent) followed by a main statement using the “IN”-Operator to filter for the CTE’s output.

Roughly looks like this:

WITH cte_table AS (...)

SELECT (stuff)

FROM tables JOIN more tables

WHERE ID IN

(SELECT ID FROM cte_table)

The CTE runs fast by itself. The main query runs fast if fed with the CTE’s output manually. Fast means that it takes no noticeable time. But when I combine both it takes almost 20 seconds.

Any ideas for solutions? Or, what is probably more important to me, the reason why it could take that long?

You could try refactoring the query to use equivalent, but different, syntax.

IN is more-or-less an inner join. So your query could become

...
FROM cte_table JOIN tables JOIN more tables
...

For your query a row has to be in the CTE output to be returned. The same applies to this inner join syntax. The two queries are equivalent.

Although you and I know these two are interchangeable the query optimizer doesn’t always realize this. So by giving it a different place to start from you can (sometimes) end up with a different plan.

Really need to see the table definitions and the “slow” plan for a proper answer, though.