Question

In SQL Server 2005 I have encountered a table with unique ID column (with unique index on it) and primary key clustered index on it too (so there are explicitly 2 indexes on this column). Is this a major performance factor on insert/update/delete on this table?

I’m trying to boost performance on a database created long ago and I wonder if removing such reduntant unique indexes could help. Does the database check/rebuild both of these indexes on every content modification? Or would the performance gain be too small to even bother with this?

Here is a sample index usage output:

INDEX   UserSeeks    UserScans  UserLookups UserUpdates
--------------------------------------------------------
1_PK    45517046      42911     245353       0
1_UQ    45517046      42911     245353       0
1_Other 45517046      42911     245353       0   
--------------------------------------------------------
2_PK    21538111      5685      231030      1121
2_UQ    21538111      5685      231030      1121
3_other 21538111      5685      231030      1121

And here is the query I used to get that data:

SELECT OBJECT_NAME(I.OBJECT_ID) AS ObjectName,
   I.NAME AS IndexName,
   S.user_seeks AS UserSeeks,
   S.user_scans AS UserScans,
   S.user_lookups AS UserLookups,
   S.user_updates AS UserUpdates
FROM sys.indexes I 
JOIN sys.dm_db_index_usage_stats S
  ON (S.OBJECT_ID = I.OBJECT_ID)
WHERE(database_id = DB_ID())

And fixed join condition:

SELECT OBJECT_NAME(I.OBJECT_ID) AS ObjectName,
   I.NAME AS IndexName,
   S.user_seeks AS UserSeeks,
   S.user_scans AS UserScans,
   S.user_lookups AS UserLookups,
   S.user_updates AS UserUpdates
FROM sys.indexes I 
JOIN sys.dm_db_index_usage_stats S
  ON (S.OBJECT_ID = I.OBJECT_ID)
  AND(S.index_id = I.index_id)
WHERE(database_id = DB_ID())

Answer 1

That might not be a redundant index.

Having a much narrower non clustered index which just contains the IDs may well have been put there as a deliberate strategy to benefit certain queries and/or to make foreign key validation more efficient.

I suggested that approach in my answer here to (successfully) resolve a deadlock problem the OP was having.