Clustered and covering index ignored on delete statement. Table scan occurs

Question

Why would SQL Server 2005 find it more efficient to perform a table scan instead of using the available clustered index on the primary key (and only the primary key)?

DISCLAIMER: There is also a non-clustered, non-unique index on the primary key with no included columns. This is baffling to me and we've had a good office chuckle already. If this index ends up being the problem, then we know who to shoot. Unfortunately, it's a production site and I can't just rip it out but will make plans to do so if necessary.

Maybe the problem is not the mentally deficient contrary index, however...

According to FogLight PASS the following statement has been causing a scan on a table with ~10 million rows about 600 times an hour when we delete a row by the primary key:

DELETE FROM SomeBigTable WHERE ID = @ID

The table DDL:

CREATE TABLE [SomeBigTable]
(
    [ID] [int] NOT NULL,
    [FullTextIndexTime] [timestamp] NOT NULL,
    [FullTextSearchText] [varchar] (max) NOT NULL,
    CONSTRAINT [PK_ID] PRIMARY KEY CLUSTERED
    (
        [ID] ASC
    )
) -- ...
ON PRIMARY

The clustered index constraint in detail:

ADD CONSTRAINT [PK_ID] PRIMARY KEY CLUSTERED
(
    [ID] ASC 
) WITH  PAD_INDEX = OFF
       ,STATISTICS_NORECOMPUTE = OFF
       ,SORT_IN_TEMPDB = OFF
       ,IGNORE_DUP_KEY = OFF
       ,ONLINE = OFF
       ,ALLOW_ROW_LOCKS = ON
       ,ALLOW_PAGE_LOCKS = ON
       ,FILLFACTOR = 75
ON PRIMARY

The non-unique, non-clustered index on the same table:

CREATE NONCLUSTERED INDEX [IX_SomeBigTable_ID] ON [SomeBigTable]
(
    [ID] ASC
) WITH  PAD_INDEX = OFF
       ,STATISTICS_NORECOMPUTE = OFF
       ,SORT_IN_TEMPDB = OFF
       ,IGNORE_DUP_KEY = OFF
       ,ONLINE = OFF
       ,ALLOW_ROW_LOCKS = ON
       ,ALLOW_PAGE_LOCKS = ON
       ,FILLFACTOR = 98
ON PRIMARY

There is also a foreign key constraint on the [ID] column pointing to an equally large table.

The 600 table scans are about ~4% of the total delete operations per hour on this table using the same statement. So, not all executions of this statement cause a table scan.

It goes without saying, but saying it anyway...this is a lot of nasty I/O that I'd like to send packing.

Is sql server scanning the clustered or non-clustered index? — Brad Ingram
I don't think sql should ever do a table scan as long as there is a clustered index on the table. It should scan the clustered index. Maybe somebody dropped the clustered index and turned the table into a heap? — Brad Ingram
This would be misreporting by FogLight if that's the case. The clustered index if definitely still there. — royalt

Brad Ingram Brad Ingram · Accepted Answer · 2012-11-16T20:58:34

Have you tried recomputing statistics on the table and clearing your proc cache? e.g. something like this:

USE myDatabase;
GO

UPDATE STATISTICS SomeBigTable;
GO

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS

It could be that sql server is just using the wrong index because it has a bad plan cached from when there was different data in the table.

Clustered and covering index ignored on delete statement. Table scan occurs

2 Answers