Re: Interesting fail when migrating Pg from Ubuntu Bionic to Focal

Mladen Gogala <gogala.mladen@gmail.com> writes:
> On 3/4/22 17:03, Tom Lane wrote:
>> Mmm ... it might have just been that the planner chose not to use
>> JIT when it thought there were fewer rows involved.  Did you check
>> with EXPLAIN that these cut-down cases still used JIT?

> This is interesting and informative answer. How do I check whether JIT
> is used in the explain plan? Can you give me an example?

If EXPLAIN prints some stuff about JIT, then JIT is going to be
used, otherwise not.  Here's an example from my (entirely
unsuccessful) attempts to duplicate depesz's problem:

=# explain select p.proname, (SELECT rolname from pg_catalog.pg_roles where oid = p.proowner) from pg_proc p;
                                          QUERY PLAN
----------------------------------------------------------------------------------------------
 Seq Scan on pg_proc p  (cost=0.00..393587.22 rows=47243 width=128)
   SubPlan 1
     ->  Index Scan using pg_authid_oid_index on pg_authid  (cost=0.28..8.30 rows=1 width=64)
           Index Cond: (oid = p.proowner)
 JIT:
   Functions: 8
   Options: Inlining false, Optimization false, Expressions true, Deforming true
(7 rows)

            regards, tom lane