Query Performance with NULL

Question

I would like to know about how NULL values affect query performance in SQL Server 2005.

I have a table similar to this (simplified):

ID | ImportantDa         


        

Answer 1

Having a lot of NULLs in a column which has an index on it (or starting with it) is generally beneficial to this kind of query.

NULL values are not entered into the index, which means that inserting / updating rows with NULL in there doesn't take the performance hit of having to update another secondary index. If, say, only 0.001% of your rows have a non-null value in that column, the IS NOT NULL query becomes pretty efficient as it just scans a relatively small index.

Of course all of this is relative, if your table is tiny anyway, it makes no appreciable difference.

Answer 2

Another alternative would be two tables:

MyTable:

ID | ImportantData
------------------
1  | 'Some Text'
2  | 'Other Text'
3  | 'abcdefg'
4  | 'whatever'
5  | 'it is'
6  | 'technically'
7  | 'a varchar'
8  | 'of course'
9  | 'but that'
10 | 'is not'
11 | 'important'

QuickPicks:

MyTableID   | QuickPickOrder
--------------------------
2           | 3
4           | 4
5           | 2
8           | 1
11          | 5

SELECT   MyTable.*
FROM     MyTable JOIN QuickPicks ON QuickPickOrder.MyTableID = MyTable.ID
ORDER BY QuickPickOrder

This would allow updating QuickPickOrder without locking anything in MyTable or logging a full row transaction for that table. So depending how big MyTable is, and how often you are updating QuickPickOrder, there may be a scalability advantage.

Also, having a separate table will allow you to add a unique index on QuickPickOrder to ensure no duplication, and could be more easily scaled later to allow different kinds of QuickPicks, having them specific to certain contexts or users, etc.

Answer 3

SQL Server indexes NULL values, so this will most probably just use the Index Seek over an index on QuickPickOrder, both for filtering and for ordering.

Answer 4

NULL looks good to me as well. SQL Server has many kinds of indices to choose from. I forget which ones do this, but some only index values in a given range. If you had that kind of index on the column being tested, the NULL valued records would not be in the index, and the index scan would be fast.

Answer 5

NULL looks fine to me for this purpose. Performance is likely to be basically the same as with a non-null column and constant value, or maybe even better for filtering out all NULLs.

Answer 6

The alternative is to normalize QuickPickOrder into a table with a foreign key, and then perform an inner join to filter the nulls out (or a left join with a where clause to filter the non-nulls out).