Capturing the Execution Plan

One last post on execution plans and Profiler (at least for now)

When trying to check a query’s execution plan, the display execution plan option of Management Studio is usually adequate, however there are occasions where it’s either not feasible to run the query from Management Studio or the particular behaviour can’t be reproduced in Management Studio (perhaps because of different SET options). For cases like this it may be necessary to capture the execution plans via SQL Trace. Fortunately there are events for execution plans. Eight of them, to be precise, all under the Performance folder.

Great, so there’s no shortage of options available. But what are the differences between them?

Showplan All

According to Books Online:

The Showplan All event class occurs when Microsoft SQL Server executes an SQL statement. Include this event class to identify the Showplan operators on Microsoft SQL Server 2000 or Microsoft SQL Server 7.0. This event class will also work on SQL Server 2005 and later; however, the information included is a subset of the information available in the Showplan XML Statistics Profile or Showplan XML event class.

So in other words this one is not generally the event that we should be looking at on the latest versions of SQL. It’s not deprecated however (at least not in SQL 2008), so it is still usable on the later versions if you absolutely want.

The plan is in text format and hence it will be smaller than the XML format plan (like the one produced by Management Studio’s “include execution plan” option). Important to note is that the run-time statistics (eg “Actual row count”) are not present in this event. What is present is the text form of the plan, the estimated row counts for each operator, physical and logical operator names, any details of what the operator operated on, estimated rows, executions, IO and CPU costs, row size and some costs.

The event fires once per query executed, so if a single query is executed four times, the event will fire four times.

Also worth noting is that for all of the Showplan events, the BinaryData column must be included in the trace, as without that the plan will not be captured.

Showplan All for Query Compile

Very similar to the Showplan All event in appearance, the difference from the Showplan All event is when it fires. While Showplan All fires on query execution, Showplan All for Query Compile event fires when the query is optimised, not executed. Hence run the query four times and the Showplan All for Query Compile event will fire once, not four times (assuming that the plan is not already in cache and nothing forces a recompile on any of the executions)

Like with the Showplan event, there is no run-time information. This shouldn’t be surprising, since the event fires at query compile, not execution.

Showplan Statistics Profile

Again, very similar to the Showplan All event in appearance. As with Showplan All event, the Books Online entry indicates that this is more use against SQL 7 and 2000 servers.

This is the first event that we’ve looked at that does contain the run-time information, specifically the execution count and row count for each operator.

Showplan Text

This is the event that produced the least information of all of the plan events. Books Online has a similar, but longer, note as for Showplan All.

The Showplan Text event class occurs when Microsoft SQL Server executes an SQL statement. Include this event class to identify the Showplan operators on SQL Server 2000 or SQL Server 7.0. This event class works on SQL Server or later, however the information included is a subset of the information available in Showplan All, Showplan XML Statistics Profile or Showplan XML event class.

All that this event shows is the text form of the plan. No estimated or actual statistics are included, no costs are available, no details of operator names or details.

This may be useful if there’s a need for a long-term trace for execution plans, since it’s the smallest amount of information it will result in the smallest files and lowest impact.

Showplan Text (Unencoded)

This event shows exactly the same information as Showplan Text does. The only difference is that the plan is formatted as a string and placed into the TextData column instead of the BinaryData column as with previous events.

Showplan XML

The XML plan events are the ‘new’ events, first appearing in SQL 2005. One thing of interest with these events is that the resulting plans don’t have to be stored in the trace file, there is an option (with the profiler GUI that is) to save them into separate files. This is only an option when using the Profiler GUI, a server-side trace has no such capability.

With this event, the plan is stored in the textdata column, so the BinaryData column is not required, unlike with the Showplan All events.

Like with the Showplan All event, this event contains no run-time information. It has only the compile time information, much as the ‘Display Estimated Execution Plan’ option in Management Studio.

The event fires once per query executed, so if a single query is executed four times, the event will fire four times.

Showplan XML for Query Compile

Almost identical to the Showplan XML event, the difference is that this event fires on query compile rather than query execution. Hence run the query four times and the Showplan XML for Query Compile event will fire once, not four times.

Showplan XML Statistics Profile

This is pretty much the top of the execution plan events – the full XML execution plan, with all compile- and run-time information on each execution of the query.

Of course, as the largest of the plan events it’s going to have an impact on the server and as such should be traced with caution. It’s also going to result in large trace files if run for any length of time. Test first before running a trace against a busy production server.