When do my stored procedure execution plans get updated?

For the sake of brevity in this article I am going to group procedures, functions and routines together as stored procedures and ignore any differences between them.

What does the SQL command UPDATE STATISTICS FOR PROCEDURE/FUNCTION/ROUTINE does and perhaps, more pertinently, as a DBA do I need to run this regularly to ensure my systems are working efficiently? For those wanting an immediate answer I think it is “never” or “almost never“, the reasons for which I hope to explain clearly in this article.

The command itself is straightforward: calling it causes Informix to parse a stored procedure’s code and produce a query plan for all of it based on the current statistics and data distributions (if the procedure references any tables). It then writes the query plan to the sysprocplan table which is, unless you have an unlogged database, a logged operation written to the logical log. Used with no parameters it does this for all stored procedures in the system.

As long as the query plan in sysprocplan is reasonably efficient there is probably no need to (ever) proactively update it but there may be cases when you’d want to do so, for example, if a very small or empty table has grown into a large one. However if you were to do this your new plan would be based on the current table statistics and data distributions and if these haven’t been updated yet you may get the same, now inefficient, plan.

The manual states:

The sysprocplan system catalog table stores execution plans for SPL routines. Two actions can update the sysprocplan system catalog table:

  • Execution of an SPL routine that uses a modified table
  • The UPDATE STATISTICS FOR ROUTINE, FUNCTION, or PROCEDURE statement.

There is a created column in the sysprocplan table but it’s a date and not a date/time which makes it much harder to match plan updates to other events.

So what is a modified table? Quite simply it is one where the version number has been incremented. You can see the version number with an SQL query like:

select version from systables where owner='myowner' and tabname='mytable';

I think the only reference to this in manual is in section about the systables view where it simply says:

version
INTEGER
Number that changes when table is altered

How the engine works out the dependencies a stored procedure has on different tables falls into the category of system internals, which IBM chooses not to publicly document, but I think it’s safe to say that if a table is referenced anywhere in a procedure it is dependent on it.

There are many ways a table can be “altered”, some more obvious than others:

Method Version number incremented by
GRANT SELECT 1
GRANT UPDATE 1
GRANT SELECT 1
GRANT SELECT, UPDATE 1
UPDATE STATISTICS HIGH 2
UPDATE STATISTICS MEDIUM 2
UPDATE STATISTICS [LOW] 2
RENAME COLUMN 65536
CREATE INDEX 65536
ADD column 131072
DROP column 131072
GRANT CONNECT 0
GRANT RESOURCE 0
GRANT DBA 1

I am not sure why some operations increment the value by large numbers, all powers of 2, as any increment has a similar effect, at least as far as the scope of this article is concerned.

The table is not a complete list because there are many possible DDL operations but this does already illustrate or suggest that:

  • On most systems it’s likely that UPDATE STATISTICS commands will be the main trigger for stored query plans to be updated. If you run LOW, HIGH and MEDIUM modes for a table like you will if you use AUS or dostats, you’ll trigger at least three updates for dependent stored procedures (if they are called).
  • If we want to grant multiple privileges on the same table, it’s best to do it in a single statement because if a dependent stored procedure is being called in between running commands by an application, its stored execution plan will be updated only once.
  • GRANT DBA is not a table level operation yet it has an effect.

Further testing shows that both the GRANT DBA and REVOKE DBA statements increment the version number on all tables in the same database where the tabid is 100 or greater, that is all user tables. From the manual above it follows that the stored query plans for all stored procedures or functions dependent on a table will be updated the next time they are executed.

On our systems we see a large amount of writes to sysprocplan after granting or revoking the DBA privilege to anyone. When graphed we see a sharp peak and a long exponential tail off as less commonly used procedures get called.

Therefore if you grant DBA to a user on a busy live system, it can affect concurrency more than you might expect. On an idle system you may want to run UPDATE STATISTICS FOR PROCEDURE immediately afterwards to update the stored query plans in an orderly way and save the first session to call any given procedure from this overhead.

I think running the command offline to avoid the overhead for user or application sessions is possibly the only true use case for this command.