SQL-或导致查询运行极其缓慢

Question

When I combine two conditions in the WHERE clause of my query, it takes well over a minute to run. If I split into two different queries, they both run in about 1 second.

Any thoughts? See below:

This takes well over a minute

SELECT 
    COUNT(b.BookKey)
from
    Books b (nolock)
    inner join BookPublishCities bp (nolock)
        on b.BookKey = bp.BookKey
where
    contains(bp.PublishRegionName, @SearchTerm) OR contains(b.BookTitle, @SearchTerm)

Together these are about 1 second

SELECT 
    COUNT(b.BookKey)
from
    Books b (nolock)
    inner join BookPublishCities bp (nolock)
        on b.BookKey = bp.BookKey
where
    contains(bp.PublishRegionName, @SearchTerm)

-- and...

SELECT 
    COUNT(b.BookKey)
from
    Books b (nolock)
    inner join BookPublishCities bp (nolock)
        on b.BookKey = bp.BookKey
where
    contains(b.BookTitle, @SearchTerm)

• There is a full-text index on Books.BookTitle and BookPublishCities.PublishRegionName.
• Both tables contain about 500k rows. Reason I didn't include PublishRegionName in Books table is because you're only allowed one full-text index per table.
• There is an index on BookPublishCities.BookKey

I'm just not sure why separate queries are so much faster. Thoughts?

Answer 1

or can pose a problem for SQL optimizers. Usually, this problem arises with join conditions, but it may also be happening here. Try using union and see if this is faster:

SELECT COUNT(*)
FROM ((SELECT bookkey
       FROM BookPublishCities bp (nolock)
       WHERE contains(bp.PublishRegionName, @SearchTerm)
      ) UNION ALL
      (SELECT BookKey
       FROM Books b (nolock)
       WHERE contains(b.BookTitle, @SearchTerm)
      )
     ) b;

Note that I simplified the subqueries. The join s don't seem necessary given the logic -- although under some circumstances they might be important (for instance, if the joins are used for filtering because book keys are not in both tables).

This also might not return exactly the same results, because presumably a title and a region could both match. This could also generate duplicates in your query, so you might want count(distinct) in the outer query.

Answer 2

Set SHOWPLAN_ALL on . Inspect the plan for the combination and compare it to the individuals. You may need to provide some hints to the QEP