Tuesday, October 14, 2008

Online indexes in SQL Server

Do you know that normally when you create an index on an existing table, you can not perform any query or updating while creating index on a table. This has always been like that before SQL Server 2005. With SQL Server 2005, Microsoft introduced ONLINE indexes in SQL Server. It must be known that online does not mean faster index creation but quite the contrary it is slower than its other counterpart.

In order to allow modification and querying data during the index creation, SQL Server maintains versions of rows in a Version Store. You can check size of version store by using VERSION STORE SIZE counter in performance monitor.

Now you have two options for creating indexes. They are as follows:
1. Online
2. Offline

It must be remembered that online indexes are only available in Enterprise edition of SQL Server.

With offline indexes any DML operations is not allowed. This is done by acquiring EXCLUSIVE lock on the table or indexed view. So for the whole duration of creation of index, no DML operation (including select) is allowed on the object.

There are three phases of creation of an online index. They are as follows:

1. Preparation: In preparation phase, following tasks are carried out:
a. Acquiring of lock mainly S and IS lock on table.
b. Creation of metadata for index.
c. Recompiling DML plans by incrementing version number.

2. Build

3. Final

For creating an index there must be sufficient temporary space available in disk. This space is required for sorting and other intermediate tasks. Extra space is used from disk when SQL Server does not find necessary space in primary memory to hold the temporary data. When SQL Server is using the disk space, it may acquire the space from user database if SORT_IN_TEMPDB option is not set for index creation or modification. There are certain advantages for using tempdb for this purpose. With this option most contiguous space is allocated for index which makes queries faster on the index. Additionally it uses tempdb transaction log.

When creating an index it is also to be made sure that transaction log has sufficient disk space as indexing creating large data loads which might fill the transaction log. It is best if you change the recovery model to SIMPLE or BULK_LOGGED before starting the index operation.

If an index is fragmented then it is either reorganized or rebuilt with ALTER INDEX REORGANIZE or ALTER INDEX REBUILD command. It must be remembered that reorganizing an index is always an online operation but rebuilding an index can be online or offline. For rebuild an index is dropped and a new index is created.

Index creation can also be further controlled with providing hints for maximum degree of parallelism in the CREATE or ALTER index statement. But keep in mind that as the degree of parallelism increases the requirement of memory also increases. So make sure that you have enough memory available.

ON MyDB.MyTable (MyID)

Note: MAXDOP option can not be specified in ALTER INDEX REORGANIZE statement. Currently, it is always a single threaded operation. You can also specify MAXDOP in sp_configure command. This would apply the parallelism setting on all operations of the server instance.

No comments: