SQL Server 2005-选择性能-WHERE和NULL

Question

I have the following issue:

SQL Server 2005

• Table: Documents
• Records: 400'000
• Field Value IS NOT an INDEX

• Query:

   SELECT * FROM Documents WHERE value = 0 

CASE A:

• ALL 400'000 Records have value = NULL
• Performance = X Seconds

CASE B:

• 200'000 Records have value = NULL
• 200'000 Records have value <> NULL
• Performance = X + Y Seconds

CASE C:

• 400'000 Records have value <> NULL
• Performance = X + Y + Z Seconds

Is this a common behavior, or do I have to search the cause of the performance degrees somewhere else ?

Answer 1

You almost certainly don't have an index on the value column. Given this assumption, your results aren't very surprising. NULL is internally stored as a separate bit that is simpler to lookup (and reject) than performing a numeric comparison.

If you had an index, then I'd be somewhat suprised by these results. It would also help if you posted exact figures about the table (column widths), as well as exactly what results you saw instead of "X, Y and Z". And as always, when checking for performance issues look at the query plan .