Badly performing query plan after adding ORDER BY to Full Text CONTAINS search
-
16-10-2019 - |
Question
The following query is proving beyond my abilities to fix - hope someone can help.
SELECT top 100 Id, Name, InsertDate
FROM Foo
WHERE CONTAINS(FooText, '"test*" and "hello*" and "goodbye*"')
ORDER BY InsertDate desc
Without the ORDER BY clause, the query performs fine (< 2 seconds). But when I add the ORDER BY, performance is awful (> 2 minutes).
The problem seems to be an incorrect query plan.
The query plan that performs well is this:
The query plan that performs badly is this:
IX_Foo_InsertDate is defined as: CREATE CLUSTERED INDEX [IX_Foo_InsertDate] ON [Foo] ( [InsertDate] DESC )
What is causing the change in query plans? How can I fix it?
Solution
The optimizer is choosing the different plan because it thinks the benefit of having the pre-sorted InsertDate
(using IX_Foo_InsertDate
) outweighs the cost once you add the order by
clause.
The optimizer cannot be perfect, it might choose a slower path even if statistics are accurate - but it will certainly make poor choices if the statistics are not accurate - are AUTO_CREATE_STATISTICS and AUTO_UPDATE_STATISTICS on?
If necessary, you might be able to 'trick' the optimizer like this:
SELECT top 100 Id, Name, InsertDate
FROM Foo
WHERE CONTAINS(FooText, '"test*" and "hello*" and "goodbye*"')
ORDER BY some_function_of(InsertDate) desc
where some_function_of(InsertDate)
has the same order as InsertDate