Will more indexes on a table affect performance? Possible Duplicate:
  How do indices impact query performance?
  How to know when/if I have too many indexes?  





Say ‘X’ report using some "where" condition in the statement so we are creating an Index ‘A’ on table ‘T’.
Say ‘Y’ report using some "where" condition in the statement so we are creating an Index ‘B’ on table ‘T’..
Say ‘Z’ report using some "where" condition in the statement so we are creating an Index ‘C’ on table ‘T’.
Say ‘M’ report using some "where" condition in the statement so we are creating an Index ‘D’ on table ‘T’.


Now there are 4 indexes and this table has a huge volume of records.


When I insert or update rows in this table, do the indexes affect the performance of those operations?
If so, then how do I tune those queries to perform well?
An INSERT will touch the clustered index, or heap, and every nonclustered index.  An UPDATE will touch the clustered index, or heap, and every nonclustered index that contains, at either the key or the leaf level, the column being updated.  A DELETE will touch the clustered index, or heap, and every nonclustered index.  An exception to these statements is when a record is filtered out of the index.

A general rule of thumb is that the more indexes you have on a table, the slower INSERT, UPDATE, and DELETE operations will be.  This is why adding indexes for performance is a trade off, and must be balanced properly.

Hope this helps,

Matt