Finding Implicit Column Conversions in the Plan Cache

Last year Tibor Karaszi posted a blog post titled Match Those Types that talked about implicit data type conversions associated with mismatched data types between filter criteria and table column data types.  His blog post does a good job of covering the topic so I am not going to rehash it out here.

However, at the time I was working on a number of scripts that query information from the plan cache in SQL Server 2005 and 2008 for a presentation I had in mind but never got around to actually doing.  I mentioned this in the comments on the blog post and yesterday I was contacted by email asking if I would share the script.  I dug it out, and it wasn’t exactly complete, but I decided to send what I had to the requestor any way. 

Being the person I am, I was somewhat bothered by the half completed script so I put a little time (ok, so in reality was a lot more than I originally planned to) into finishing the script off to where I originally wanted it to be.

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

DECLARE @dbname SYSNAME
SET @dbname = QUOTENAME(DB_NAME());

WITH XMLNAMESPACES
   (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan')
SELECT
   stmt.value('(@StatementText)[1]', 'varchar(max)'),
   t.value('(ScalarOperator/Identifier/ColumnReference/@Schema)[1]', 'varchar(128)'),
   t.value('(ScalarOperator/Identifier/ColumnReference/@Table)[1]', 'varchar(128)'),
   t.value('(ScalarOperator/Identifier/ColumnReference/@Column)[1]', 'varchar(128)'),
   ic.DATA_TYPE AS ConvertFrom,
   ic.CHARACTER_MAXIMUM_LENGTH AS ConvertFromLength,
   t.value('(@DataType)[1]', 'varchar(128)') AS ConvertTo,
   t.value('(@Length)[1]', 'int') AS ConvertToLength,
   query_plan
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY sys.dm_exec_query_plan(plan_handle) AS qp
CROSS APPLY query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple') AS batch(stmt)
CROSS APPLY stmt.nodes('.//Convert[@Implicit="1"]') AS n(t)
JOIN INFORMATION_SCHEMA.COLUMNS AS ic
   ON QUOTENAME(ic.TABLE_SCHEMA) = t.value('(ScalarOperator/Identifier/ColumnReference/@Schema)[1]', 'varchar(128)')
   AND QUOTENAME(ic.TABLE_NAME) = t.value('(ScalarOperator/Identifier/ColumnReference/@Table)[1]', 'varchar(128)')
   AND ic.COLUMN_NAME = t.value('(ScalarOperator/Identifier/ColumnReference/@Column)[1]', 'varchar(128)')
WHERE t.exist('ScalarOperator/Identifier/ColumnReference[@Database=sql:variable("@dbname")][@Schema!="[sys]"]') = 1

This queries the plan cache for query plans from the current database that have an implicit conversion on the column side of the query and returns the statement from the batch that is causing the conversion, the schema, tablename, and columnname being converted, as well as the original and converted datatypes and lengths and finally the entire query plan so that you can see whether it is an adhoc statement causing the conversion or if it is occurring inside of a stored procedure.

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