Blog Post

Querying the Plan Cache

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I love working with indexes, and I need to know what’s using them to work on them intelligently. Most of that information is already there waiting for you to query it. Luckily, Jonathan Kehayias (b|t) did the hard work for us in his post Finding what queries in the plan cache use a specific index, and I could modify his code to get entire tables.

Remember that you’re querying XML and that’s a CPU intensive process. However, you’re also looking for what’s in cache which is most relevant during or just after your busiest time of day on prod. The longer you wait, the more chance a query will be flushed from cache for one reason or another, although waiting a couple hours typically isn’t a problem on a server that’s not under extreme stress.

This first script will find all references to the indexes of a table in the plan cache, including key and RID lookups. However, table scans against heaps are in the XML a little different, and that’s what my second query is for. Hopefully you have a clustered index on every table you want to run this against, but you’ll probably need both of these.

If you’re only looking for a single index in the cache, I already have that query on my Cleaning up the buffer pool post. However, I’m working on some new stuff that’s way too long to put scripts in-line, so I had to create this post to add as a reference. I’ll try to remember to update this post as I have everything done, but keep an eye out for my “Indexing Strategy” post and my presentation by the same name I hope to debut at the Cleveland SQL Saturday in February 2016.

Table Index Usage

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
DECLARE @TableName SYSNAME = '[ShiverMeTuples]'; 
DECLARE @DatabaseName SYSNAME;
 
SELECT @DatabaseName = '[' + DB_NAME() + ']';
 
WITH XMLNAMESPACES
   (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan')
SELECT
    n.value('(@StatementText)[1]', 'VARCHAR(4000)') AS sql_text,
    --n.query('.'),
    cp.plan_handle,
    i.value('(@PhysicalOp)[1]', 'VARCHAR(128)') AS PhysicalOp,
    i.value('(@EstimateRows)[1]', 'VARCHAR(128)') AS EstimateRows,
    i.value('(@EstimateIO)[1]', 'VARCHAR(128)') AS EstimateIO,
    i.value('(@EstimateCPU)[1]', 'VARCHAR(128)') AS EstimateCPU,
cp.usecounts,
    i.value('(./IndexScan/@Lookup)[1]', 'VARCHAR(128)') AS IsLookup,
    i.value('(./IndexScan/Object/@Database)[1]', 'VARCHAR(128)') AS DatabaseName,
    i.value('(./IndexScan/Object/@Schema)[1]', 'VARCHAR(128)') AS SchemaName,
    i.value('(./IndexScan/Object/@Table)[1]', 'VARCHAR(128)') AS TableName,
    i.value('(./IndexScan/Object/@Index)[1]', 'VARCHAR(128)') as IndexName,
    --i.query('.'),
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./OutputList/ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS output_columns,
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./IndexScan/SeekPredicates/SeekPredicateNew//ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS seek_columns,
    RIGHT(i.value('(./IndexScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'), len(i.value('(./IndexScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)')) - charindex('.', i.value('(./IndexScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'))) as Predicate,
    query_plan
FROM (  SELECT plan_handle, query_plan
        FROM (  SELECT DISTINCT plan_handle
                FROM sys.dm_exec_query_stats WITH(NOLOCK)) AS qs
        OUTER APPLY sys.dm_exec_query_plan(qs.plan_handle) tp
      ) as tab (plan_handle, query_plan)
INNER JOIN sys.dm_exec_cached_plans AS cp 
    ON tab.plan_handle = cp.plan_handle
CROSS APPLY query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/*') AS q(n)
CROSS APPLY n.nodes('.//RelOp[IndexScan/Object[@Table=sql:variable("@TableName") and @Database=sql:variable("@DatabaseName")]]' ) as s(i)
--WHERE i.value('(./IndexScan/@Lookup)[1]', 'VARCHAR(128)') = 1
ORDER BY 3 
OPTION(RECOMPILE, MAXDOP 4);

Table (Heap) Scans

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
DECLARE @TableName SYSNAME = '[ItsAHeapOfSomething]'; 
DECLARE @DatabaseName SYSNAME;
 
SELECT @DatabaseName = '[' + DB_NAME() + ']';
 
WITH XMLNAMESPACES
   (DEFAULT 'http://schemas.microsoft.com/sqlserver/2004/07/showplan')
SELECT
    n.value('(@StatementText)[1]', 'VARCHAR(4000)') AS sql_text,
    n.query('.'),
    cp.plan_handle,
    i.value('(@PhysicalOp)[1]', 'VARCHAR(128)') AS PhysicalOp,
    i.value('(./TableScan/@Lookup)[1]', 'VARCHAR(128)') AS IsLookup,
    i.value('(./TableScan/Object/@Database)[1]', 'VARCHAR(128)') AS DatabaseName,
    i.value('(./TableScan/Object/@Schema)[1]', 'VARCHAR(128)') AS SchemaName,
    i.value('(./TableScan/Object/@Table)[1]', 'VARCHAR(128)') AS TableName,
    --i.value('(./TableScan/Object/@Table)[1]', 'VARCHAR(128)') as TableName,
    i.query('.'),
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./OutputList/ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS output_columns,
    STUFF((SELECT DISTINCT ', ' + cg.value('(@Column)[1]', 'VARCHAR(128)')
       FROM i.nodes('./TableScan/SeekPredicates/SeekPredicateNew//ColumnReference') AS t(cg)
       FOR  XML PATH('')),1,2,'') AS seek_columns,
    RIGHT(i.value('(./TableScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'), len(i.value('(./TableScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)')) - charIndex('.', i.value('(./TableScan/Predicate/ScalarOperator/@ScalarString)[1]', 'VARCHAR(4000)'))) as Predicate,
cp.usecounts,
    query_plan
FROM (  SELECT plan_handle, query_plan
        FROM (  SELECT DISTINCT plan_handle
                FROM sys.dm_exec_query_stats WITH(NOLOCK)) AS qs
        OUTER APPLY sys.dm_exec_query_plan(qs.plan_handle) tp
      ) as tab (plan_handle, query_plan)
INNER JOIN sys.dm_exec_cached_plans AS cp 
    ON tab.plan_handle = cp.plan_handle
CROSS APPLY query_plan.nodes('/ShowPlanXML/BatchSequence/Batch/Statements/*') AS q(n)
CROSS APPLY n.nodes('.//RelOp[TableScan/Object[@Table=sql:variable("@TableName") and @Database=sql:variable("@DatabaseName")]]' ) as s(i)
--WHERE i.value('(./TableScan/@Lookup)[1]', 'VARCHAR(128)') = 1
OPTION(RECOMPILE, MAXDOP 4);

Filed under: General Chat, Indexes, SQL Server, Tuning

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