Is this index on a table necessary? - sql

Question

I have a table with 5 indexes in SQL server. I'm aware of the fact that indexes affect inserts so I'd like to keep them to the minimum. I definitely need the first four indexes (as seen in the below sample).
However, I'm note quite sure if the last index (TimeSubmitted) is absolutely necessary - note that there is already CliendId+TimeSubmitted index.
The only reason why it is there is to make purging of expired rows from the table more efficient - or at least this is what my intention is. The purging job will be scheduled to run once a day - most likely at night.
There could be hundreds of thousands of records in the table at any given time.
Stored proc to purge the table:
CREATE PROCEDURE uspPurgeMyTable
(
#ExpiryDate datetime
)
AS
BEGIN
DELETE FROM MyTable
WHERE TimeSubmitted < #ExpiryDate;
END
Table (non relevant columns omitted):
CREATE TABLE MyTable (
[ClientId] [char](36) NOT NULL,
[UserName] [nvarchar](256) NOT NULL,
[TimeSubmitted] [datetime] NOT NULL,
[ProviderId] [uniqueidentifier] NULL,
[RegionId] [int] NULL
) ON [PRIMARY]
CREATE NONCLUSTERED INDEX [IX_MyTable_ClientArea] ON [dbo].[MyTable]
(
[ClientId] ASC,
[RegionId] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF) ON [PRIMARY]
CREATE NONCLUSTERED INDEX [IX_MyTable_ClientPrinter] ON [dbo].[MyTable]
(
[ClientId] ASC,
[ProviderId] DESC
)WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF) ON [PRIMARY]
CREATE NONCLUSTERED INDEX [IX_MyTable_ClientTime] ON [dbo].[MyTable]
(
[ClientId] ASC,
[TimeSubmitted] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF) ON [PRIMARY]
CREATE NONCLUSTERED INDEX [IX_MyTable_ClientUser] ON [dbo].[MyTable]
(
[ClientId] ASC,
[UserName] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX [IX_MyTable_TimeSubmitted] ON [dbo].[MyTable]
(
[TimeSubmitted] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF) ON [PRIMARY]

Answer 1

Yes, the IX_MyTable_TimeSubmitted index will be beneficial for the purge process you described. Since the purge is based on the TimeSubmitted column only and none of the other indexes start with that, the best SQL Server would be able to do is use them in an index scan.
As with any indexing, there are trade-offs between read and update performance. You should measure your read, insert, and purge performance with and without the additional index to see what provides the best response time for each situation. Batch processes such as the purge that run overnight may not be as time sensitive as inserting or reading in your particular scenario.

Answer 2

If you are concerned about performance, you should take a different approach, partitioning. A reasonable place to learn about partitioning is the documentation.
A partitioned table stores each partition in a separate set of files. These are invisible to users of the database, in general. However, if you want to drop old data, you can just drop a partition.
This not only eliminates the need for the index. It also eliminates the need for the delete. And, dropping a partition is much more efficient than deleting, because there is much less logging.