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

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  1. POClustered and covering index ignored on delete statement. Table scan occurs
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    <p><strong>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)?</strong> </p> <p><strong>DISCLAIMER</strong>: There is <em>also</em> 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. </p> <p>Maybe the problem is not the mentally deficient contrary index, however...</p> <p>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:</p> <pre><code>DELETE FROM SomeBigTable WHERE ID = @ID </code></pre> <p>The table DDL:</p> <pre><code>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 </code></pre> <p>The clustered index constraint in detail:</p> <pre><code>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 </code></pre> <p>The non-unique, non-clustered index on the same table:</p> <pre><code>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 </code></pre> <p>There is <em>also</em> a foreign key constraint on the [ID] column pointing to an equally large table.</p> <p>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.</p> <p>It goes without saying, but saying it anyway...this is a lot of nasty I/O that I'd like to send packing.</p>
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USJørgen R
    1. USroyalt
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
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    2. PO
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    1. VO
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      1. POClustered and covering index ignored on delete statement. Table scan occurs
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
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      1. POClustered and covering index ignored on delete statement. Table scan occurs
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
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      1. POClustered and covering index ignored on delete statement. Table scan occurs
      1. This table or related slice is empty.
      1. VTUpMod
    1. COIs sql server scanning the clustered or non-clustered index?
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      1. POClustered and covering index ignored on delete statement. Table scan occurs
      1. USBrad Ingram
    2. CONeither. It's scanning the table.
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      1. POClustered and covering index ignored on delete statement. Table scan occurs
      1. USroyalt
    3. COI 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?
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      1. POClustered and covering index ignored on delete statement. Table scan occurs
      1. USBrad Ingram
 

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