Re: Normalization of utility queries in pg_stat_statements

On Wed, Feb 08, 2023 at 12:05:24PM +0900, Michael Paquier wrote:
> Thoughts and comments are welcome. 0001 and 0002 are useful on their
> own to keep track of utilities that use Const and A_Const after going
> through the query jumbling, even if an approach based on query string
> or the automated query jumbling for utilities is used (the query
> string approach a bit its value). I'll add that to the next commit
> fest.

While wondering about this stuff about the last few days and
discussing with bertrand, I have changed my mind on the point that
there is no need to be that aggressive yet with the normalization of
the A_Const nodes, because the query string normalization of
pg_stat_statements is not prepared yet to handle cases where a A_Const
value uses a non-quoted value with whitespaces. The two cases where I
saw an impact is on the commands that can define an isolation level:
SET TRANSACTION and BEGIN.

For example, applying normalization to A_Const nodes does the
following as of HEAD:
1) BEGIN TRANSACTION READ ONLY, READ WRITE, DEFERRABLE, NOT DEFERRABLE;
BEGIN TRANSACTION $1 ONLY, $2 WRITE, $3, $4 DEFERRABLE
2) SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET TRANSACTION ISOLATION LEVEL $1 COMMITTED

On top of that, specifying a different isolation level may cause these
commands to be grouped, which is not really cool. All that could be
done incrementally later on, in 17~ or later depending on the
adjustments that make sense.

Attached is an updated patch set. 0003 is basically the same as v3,
that I have kept around for clarity in case one wants to see the
effect of a A_Const normalization to all the related commands, though
I am not proposing that for an upstream integration. 0002 has been
completed with a couple of commands to track all the commands with
A_Const, so as we never lose sight of what happens. 0004 is what I
think could be done for PG16, where normalization affects only Const.
At the end of the day, this reflects the following commands that use
Const nodes because they use directly queries, so the same rules as
SELECT and DMLs apply to them:
- DECLARE
- EXPLAIN
- CREATE MATERIALIZED VIEW
- CTAS, SELECT INTO

Comments and thoughts welcome.
Thanks,
--
Michael