Which is better for performance, make unused columns null or fill in with values

Question

I have a table that has a uniqueidentifier column only used by some of the rows.

Is it a good idea to:

make that column nullable and make unused rows' uniqueidentifier null?
or not make the column nullable and fill in a GUID value for all rows?

Will there be any performance difference with SELECT queries on the uniqueidentifier column? And what about disk space? Do null values take up the same amount of space?

possible duplicate of [How much size "Null" value takes in SQL Server](http://stackoverflow.com/questions/3731172/how-much-size-null-value-takes-in-sql-server) — LittleBobbyTables - Au Revoir, Nov 16 '12 at 20:14
@littlebobbytables that answer didn't talk about uniqueidentifiers — James, Nov 16 '12 at 20:15
Sorry, I was going off of the title of your question. You've got like 5 questions here. — LittleBobbyTables - Au Revoir, Nov 16 '12 at 20:17
@James sure it does you just need to know if a GUID is fixed or variable. I'm pretty sure its fixed to 16 bytes — Conrad Frix, Nov 16 '12 at 20:17
@LittleBobbyTables - Completely unrelated, but I love the name. XKCD ftw. — zimdanen, Nov 16 '12 at 21:29

d89761 · Accepted Answer · 2012-11-16T21:08:45.217

7

If you are on SQL Server 2008+, you might want to look into using sparse columns. It is optimized for storing a column that contains mostly null values.

edited Nov 16 '12 at 21:08

answered Nov 16 '12 at 20:57

d89761

1,434
9
11

score 0 · Answer 2 · answered Nov 17 '12 at 20:13

0

Also check MSSQL filtered indexes if you are going to use nullable columns in where clauses.

answered Nov 17 '12 at 20:13

user1832467

31
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Which is better for performance, make unused columns null or fill in with values

2 Answers2