On Mon, Jun 13, 2016 at 03:42:49PM -0400, Tom Lane wrote:
> I wrote:
> > ... there was also an unexplainable plan change:
>
> > *** /home/postgres/pgsql/src/test/regress/expected/aggregates.out Thu Apr 7 21:13:14 2016
> > --- /home/postgres/pgsql/src/test/regress/results/aggregates.out Mon Jun 13 11:54:01 2016
> > ***************
> > *** 577,590 ****
>
> > explain (costs off)
> > select max(unique1) from tenk1 where unique1 > 42000;
> > ! QUERY PLAN
> > ! ---------------------------------------------------------------------------
> > ! Result
> > ! InitPlan 1 (returns $0)
> > ! -> Limit
> > ! -> Index Only Scan Backward using tenk1_unique1 on tenk1
> > ! Index Cond: ((unique1 IS NOT NULL) AND (unique1 > 42000))
> > ! (5 rows)
>
> > select max(unique1) from tenk1 where unique1 > 42000;
> > max
> > --- 577,588 ----
>
> > explain (costs off)
> > select max(unique1) from tenk1 where unique1 > 42000;
> > ! QUERY PLAN
> > ! ----------------------------------------------------
> > ! Aggregate
> > ! -> Index Only Scan using tenk1_unique1 on tenk1
> > ! Index Cond: (unique1 > 42000)
> > ! (3 rows)
>
> > select max(unique1) from tenk1 where unique1 > 42000;
> > max
>
> > I would not be surprised at a change to a parallel-query plan, but there's
> > no parallelism here, so what happened? This looks like a bug to me.
> > (Also, doing this query without COSTS OFF shows that the newly selected
> > plan actually has a greater estimated cost than the expected plan, which
> > makes it definitely a bug.)
>
> I looked into this and found that the costs are considered fuzzily the
> same, and then add_path prefers the slightly-worse path on the grounds
> that it is marked parallel_safe while the MinMaxAgg path is not. It seems
> to me that there is some fuzzy thinking going on there. On exactly what
> grounds is a path to be preferred merely because it is parallel safe, and
> not actually parallelized? Or perhaps the question to ask is whether a
> MinMaxAgg path can be marked parallel-safe.
[Action required within 72 hours. This is a generic notification.]
The above-described topic is currently a PostgreSQL 9.6 open item ("consider
whether MinMaxAggPath might fail to be parallel-safe"). Robert, since you
committed the patch believed to have created it, you own this open item. If
some other commit is more relevant or if this does not belong as a 9.6 open
item, please let us know. Otherwise, please observe the policy on open item
ownership[1] and send a status update within 72 hours of this message.
Include a date for your subsequent status update. Testers may discover new
open items at any time, and I want to plan to get them all fixed well in
advance of shipping 9.6rc1. Consequently, I will appreciate your efforts
toward speedy resolution. Thanks.
[1] http://www.postgresql.org/message-id/20160527025039.GA447393@tornado.leadboat.com
