Multiple Indexes vs Multi-Column Indexes

Question

I've just been adding an Index to a table in SQL Server 2005 and it got me thinking. What is the difference between creating 1 index and defining multiple columns over having 1 index per column you want to index.

Are there certain reasons why one should be used over the other?

For example

Create NonClustered Index IX_IndexName On TableName
(Column1 Asc, Column2 Asc, Column3 Asc)

Versus

Create NonClustered Index IX_IndexName1 On TableName
(Column1 Asc)

Create NonClustered Index IX_IndexName2 On TableName
(Column2 Asc)

Create NonClustered Index IX_IndexName3 On TableName
(Column3 Asc)

Answer 1

Yes. I recommend you check out Kimberly Tripp's articles on indexing.

If an index is "covering", then there is no need to use anything but the index. In SQL Server 2005, you can also add additional columns to the index that are not part of the key which can eliminate trips to the rest of the row.

Having multiple indexes, each on a single column may mean that only one index gets used at all - you will have to refer to the execution plan to see what effects different indexing schemes offer.

You can also use the tuning wizard to help determine what indexes would make a given query or workload perform the best.

Answer 2

If you have queries that will be frequently using a relatively static set of columns, creating a single covering index that includes them all will improve performance dramatically.

By putting multiple columns in your index, the optimizer will only have to access the table directly if a column is not in the index. I use these a lot in data warehousing. The downside is that doing this can cost a lot of overhead, especially if the data is very volatile.

Creating indexes on single columns is useful for lookup operations frequently found in OLTP systems.

You should ask yourself why you're indexing the columns and how they'll be used. Run some query plans and see when they are being accessed. Index tuning is as much instinct as science.

Answer 3

The multi-column index can be used for queries referencing all the columns:

SELECT *
FROM TableName
WHERE Column1=1 AND Column2=2 AND Column3=3

This can be looked up directly using the multi-column index. On the other hand, at most one of the single-column index can be used (it would have to look up all records having Column1=1, and then check Column2 and Column3 in each of those).

Answer 4

I agree with Cade Roux.

This particular article should get you on the right track.

http://www.sqlskills.com/blogs/kimberly/2008/04/16/IndexesInSQLServer20052008BestPracticesPart1.aspx or http://www.sqlskills.com/BLOGS/KIMBERLY/post.aspx?id=19f0ce1c-0d2f-4ad5-9b13-a615418422e0

One thing to note, clustered indexes should have a unique key(an identity column I would recommend) as the first column. Basically it helps your data insert at the end of the index and not cause lot's of disk IO and Page splits.

Secondly, if you are created other indexes on your data and they are constructed cleverly they will be reused.

e.g. imagine you search a table on three columns

state, county, zip.

you sometimes search by state only. you sometimes search by state and county. you frequently search by state, county, zip. Then an index with state, county, zip. will be used in all three of these searches.

If you search by zip alone quite a lot then the above index will not be used(by SQL Server anyway) as zip is the third part of that index and the query optimiser will not see that index as helpful.

You could then create an index on Zip alone that would be used in this instance.

I guess the answer you are looking for is that it depends on your where clauses of your frequently used queries and also your group by's.

The article will help a lot. :-)

Answer 5

One item that seems to have been missed is star transformations. Index Intersection operators resolve the predicate by calculating the set of rows hit by each of the predicates before any I/O is done on the fact table. On a star schema you would index each individual dimension key and the query optimiser can resolve which rows to select by the index intersection computation. The indexes on individual columns give the best flexibility for this.