Re: track generic and custom plans in pg_stat_statements

Overall I like the idea; adds some nice visibility to something that has
been very ephemeral in the past.

Not included in this patch and maybe for follow-up work, is this
> information a good idea also?

can be added to EXPLAIN output and perhaps pg_stat_database.
>

I could see EXPLAIN being somewhat useful (especially for non-interactive
things like auto_explain), so a weak +1 on that.

Definitely not useful for pg_stat_database IMHO.

Some quick comments on the patch:

FILE: contrib/pg_stat_statements/expected/plan_cache.out

These tests seem very verbose. Why not just prepare a simple query:

prepare foo as select $1 > 0;
execute foo(1);
...then tweak things via plan_cache_mode to ensure we test the right things?

Also would be nice to constrain the pg_stat_statement SELECTs with some
careful WHERE clauses. Would also allow you to remove the ORDER BY and the
COLLATE.

FILE: contrib/pg_stat_statements/meson.build

oldextversions should still have a trailing comma

FILE: contrib/pg_stat_statements/pg_stat_statements.c

+ if (cplan && cplan->status > PLAN_CACHE_STATUS_CUSTOM_PLAN)

Not really comfortable with using the enum like this. Better would be
explicitly listing the two states that lead to the increment. Not as good
but still better:
cplan->status >= PLAN_CACHE_STATUS_GENERIC_PLAN_BUILD

+ PlanCacheStatus status; /* is it a reused generic plan? */

The comment should be updated

FILE: contrib/pg_stat_statements/sql/plan_cache.sql

Missing a newline at the end

FILE: doc/src/sgml/pgstatstatements.sgml

+ Total number of non-utility statements executed using a generic plan

I'm not sure we need to specify non-utility here.

Cheers,
Greg

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