Store Statistics XML in database tables using SQL Traces for further analysis.

Since SQL Server 2005, query plan as well as statistics of query execution can be captured in XML format. Also, SQL Server 2005 has XQuery support to directly query XML document. By combining these two new features, users can analyze the query plans using queries.

 

However, in SQL Server, there is no easy way to capture the statitics XML into a table. Fortunately, there are SQL traces provided by SQL Server to capture the showplan XML and statistics XML information into trace files and loaded into tables.

 

Note: The only limitation is the 128 level of nesting levels supported by XML data type in SQL 2005. In that case, you have to write client code to parse the query plan, which going to be a very complex query plan.

 

Here is a small example using SQL traces to store the statistics XML and extract the estimated rows and actual rows. 

/*Using Traces to Capture Statistics XML*/

declare @trace_id int

declare @trace_file nvarchar(200)

select @trace_file = 'c:\temp\test_stats_' + cast(newid() as varchar(100))

-- using trace table.

exec sp_trace_create @trace_id output,

      2,

      @tracefile=@trace_file

-- capture statistics-xml, textdata, on

exec sp_trace_setevent @trace_id, 146, 1, 1

-- start

exec sp_trace_setstatus @trace_id, 1

-- test statement.

select * from sys.objects

-- stop

exec sp_trace_setstatus @trace_id, 0

-- close

exec sp_trace_setstatus @trace_id, 2

-- load trace files into table

if object_id('temp_trc') is not null

      drop table temp_trc

select *

into temp_trc

from fn_trace_gettable(@trace_file + '.trc', default)

-- look at the captured stats xml

declare @plan xml

select @plan=cast(textdata as xml)

from temp_trc

where eventclass = 146;

-- collect the actual and also estimate stats.

with XMLNAMESPACES ('https://schemas.microsoft.com/sqlserver/2004/07/showplan' as sql)

select

      ro.relop.value('@NodeId', 'int') NodeId,

      ro.relop.value('@PhysicalOp', 'nvarchar(200)') PhysicalOp,

      ro.relop.value('@LogicalOp', 'nvarchar(200)') LogicalOp,

      (ro.relop.value('@EstimateRows', 'float')

            * (ro.relop.value('@EstimateRewinds', 'float')

                  + ro.relop.value('@EstimateRebinds', 'float')

                  + 1.0)) EstimateRows,

      case

            when root_actual.ActualRows = 0

                  then null

            else root_actual.ActualRows

      end ActualRows,

      cast(ro.relop.exist('*/sql:RelOp') as bit) IsNotLeaf

from @plan.nodes('//sql:RelOp') as ro(relop)

      cross apply (

            select sum(rti.info.value('@ActualRows', 'float')) ActualRows

            from ro.relop.nodes('sql:RunTimeInformation/sql:RunTimeCountersPerThread') as rti(info)

      ) root_actual;

go

The output of the estimate rows and actual rows is given below:

NodeId	PhysicalOp	LogicalOp	EstimateRows	ActualRows	IsNotLeaf
0	Nested Loops	Left Outer Join	52	52	1
1	Nested Loops	Left Outer Join	52	52	1
2	Filter	Filter	52	52	1
3	Compute Scalar	Compute Scalar	52	NULL	1
4	Clustered Index Scan	Clustered Index Scan	52	52	0
13	Clustered Index Seek	Clustered Index Seek	52.000031	NULL	0
14	Clustered Index Seek	Clustered Index Seek	52	52	0

Comments

• Anonymous
  August 06, 2007
  Hi  Guys Anybody able to elaborate on how you view the histogram for multi column statistics. I have a query plan that has estimated Rows (2943) and Actual Rows (3099750). Out by a factor 1000.

• Anonymous
  November 13, 2007
  The comment has been removed