Tips, Tricks, and Advice from the SQL Server Query Processing Team : Statistics XML

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

queryproc — Fri, 01 Jun 2007 19:51:00 GMT

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,

@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 ('http://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;

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

Viewing and Interpreting XML Showplans

queryproc — Sat, 07 Oct 2006 00:03:00 GMT

As mentioned in our previous blog posting, SQL Server 2005 supports Showplan generation in XML format. XML-based Showplans provide greater flexibility in viewing query plans and saving them to files as compared to legacy Showplans. In addition to the usability benefits, XML-based Showplans also contain certain plan-specific information which is not available in Legacy Showplans. For example, Showplan XML contains the cached plan size, memory fractions (how memory grant is to be distributed across operators in the query plan), parameter list with values used during optimization, and missing indexes information which is not available with the legacy Showplan All option. Similarly Statistics XML contains additional information such as degree of parallelism, runtime memory grant, parameter list with actual values for all parameters used, and execution statistics such as count of rows/executes aggregated per thread (in a parallel query) as compared to the legacy Statistics Profile option. Such information is very useful in analyzing query compilation, execution and performance issues, hence using the new XML features for Showplan is highly recommended.

Showplan XML can be generated in two ways:

1. Using T-SQL SET options

2. Using SQL Server Profiler trace events

With the SET options, Showplan XML returns batch-level information, i.e. it produces one XML document per T-SQL batch. If the T-SQL batch contains several statements, it generates one XML node per statement and concatenates them together. However when Showplan XML is generated using trace events, it only generates statement-level information. Let’s analyze the Showplan XML output for the following query (see attached document - showplan.xml):

use nwind

set showplan_xml on

SELECT ContactName, OrderDate

FROM Customers inner join Orders

ON Orders.CustomerID = Customers.CustomerID

WHERE Orders.ShipCountry = 'Canada'

set showplan_xml off

The root element of the document contains the Showplan XML namespace attribute with the location of the Showplan XML schema, and the SQL Server build information. It contains batch and statement sub-elements. Each statement contains a "StatementText" attribute that describes the T-SQL query being executed, a "StatementId" attribute indicating the relative position of the statement in the batch, the relative cost of the statement and the level of optimization used to generate the query plan output. It also contains additional information like SET options in effect when executing the query.

<Batch>

<StmtSimple StatementText="SELECT ContactName, OrderDate FROM Customers inner join Orders ..

...

The QueryPlan node contains plan information such as size of the compiled plan, compilation time, etc. which is useful for debugging purposes. The iterators in the query plan are represented as nested elements, each of type ‘RelOp’. Every ‘RelOp’ element contains two types of information:

Generic information such as logical and physical operator names, optimizer cost estimates, etc. as attributes.
Operator specific information such as a list of output columns, a set of defined values, predicates, database objects on which they operate (tables/indexes/views), etc. as sub-elements.

For example, the topmost ‘RelOp’ in our example is a ‘Nested Loops’ that contains the following generic information:

And the following operator specific information:

</OutputList>

</OuterReferences>

…

</NestedLoops>

The runtime counterpart of Showplan XML is called ‘Statistics XML’. It displays query execution “statistics” by executing the query and aggregating runtime information on a per-iterator basis. To obtain the Statistics XML output, use the following SET option.

set statistics xml on

Now lets analyze the Statistics XML output of the above query. The XML generated is semantically similar to the Showplan XML output, some of the key differences are:

In the QueryPlan node, an additional attribute indicating the degree of parallelism is present:

Each iterator ‘RelOp’ node contains an additional sub-element called RunTimeInformation that contains iterator execution profile such as the number of rows, number of executions, indexes accessed, join order, etc.

<RelOp NodeId="0" PhysicalOp="Nested Loops" LogicalOp="Inner Join" EstimateRows="43" EstimateIO="0"

EstimateCPU="0.00017974" AvgRowSize="34" EstimatedTotalSubtreeCost="0.0308059" Parallel="0" EstimateRebinds="0" EstimateRewinds="0">

</OutputList>

<RunTimeInformation>

<RunTimeCountersPerThread Thread="0" ActualRows="43" ActualEndOfScans="1" ActualExecutions="1" />

</RunTimeInformation>

The runtime information is shown per thread, and not aggregated as in legacy showplan.
For memory consuming iterators such sort or hash join, the memory grant information is printed.

In a later blog post, i will cover some of the Profiler Trace Events for Showplan.

- Gargi Sur

SQL Server Query Processing Development Team

Introduction to Showplan

queryproc — Sat, 30 Sep 2006 01:16:00 GMT

Showplan is a feature in SQL Server to display and read query plans. While some of you may already be very familiar with Showplan, it is one of the most important diagnostic tools that we use in the query processing team to locate and identify problems, and therefore deserves some extra exposure. Being able to collect, read, and understand Showplan data is a critical skill to have, and one which we plan to blog about a fair bit. Consider this an introductory posting to cover some of the basics which we will drill into more in further posts.

We generate two types of Showplans in SQL Server, one at query compilation time (when an optimized query plan is produced) and the second at query runtime (when the optimized query plan is executed). The former allows you to see the compiled query plan in its entirety, as well as a ton of useful information about each operator in the plan. The runtime plan, often referred to as query execution “statistics”, enables you to collect actual metrics about the query during its execution, such as its execution time and actual cost.

Showplan and statistics information can be extracted in two ways from the server, 1) using query SET options, 2) using Profiler Trace events. The various SET options available are:

· Legacy Showplan

o Compile Time: SET SHOWPLAN_ALL ON or SET SHOWPLAN_TEXT ON

o Runtime: SET STATISTICS PROFILE ON

· Showplan XML (Preferred)

o Compile Time: SET SHOWPLAN_XML ON

o Runtime: SET STATISTICS XML ON

SQL Server Management Studio (SSMS) uses Showplan to display a graphical representation of the query plan. A graphical view of the query plan for the following query on the ‘nwind’ database is shown in the picture below.

-- Query: select customer name and order date for orders placed by customers in Canada

use nwind

select ContactName, OrderDate

from Customers inner join Orders

on Orders.CustomerID = Customers.CustomerID

where Orders.ShipCountry = 'Canada'

(See the attached .bmp file for a picture of this showplan)

In SSMS, to view the graphical plan, click on the Execution Plan tab in the Results Pane. The graphical plan is read from top-to-bottom, right-to-left. When you select any operator in the query tree or simply hover the mouse on it, you will see a tooltip that describes the operator. It displays the Query Optimizer cost estimates (operator and subtree costs, number of rows, row size, etc.) and additional information like output columns, predicates, etc. The detailed operator information is shown in the Properties window (View --> Properties Window), which is usually displayed on the extreme right frame in SSMS.

To save the graphical showplan to a file you can right click on the Execution Plan and select 'Save Execution Plan As'. The query plan is saved with extension '.sqlplan' and can be reloaded into SSMS anytime, sent via email, etc.

Showplan output can be generated in text, grid or XML formats. The XML format was introduced in SQL Server 2005 which we refer to as “Showplan XML” and the other formats as “Legacy Showplan”. In the next posting we will describe how to generate and analyze Showplan XML.

- Steve and Gargi