Re: Runtime pruning problem

From: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Yuzuko Hosoya <hosoya(dot)yuzuko(at)lab(dot)ntt(dot)co(dot)jp>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Runtime pruning problem
Date: 2019-04-19 08:13:43
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On 2019/04/19 17:00, Amit Langote wrote:
> On 2019/04/19 2:25, Tom Lane wrote:
>> Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> writes:
>>> Another idea is to teach explain.c about this special case of run-time
>>> pruning having pruned all child subplans even though appendplans contains
>>> one element to cater for targetlist accesses. That is, Append will be
>>> displayed with "Subplans Removed: All" and no child subplans listed below
>>> it, even though appendplans[] has one. David already said he didn't do in
>>> the first place to avoid PartitionPruneInfo details creeping into other
>>> modules, but maybe there's no other way?
>> I tried simply removing the hack in nodeAppend.c (per quick-hack patch
>> below), and it gets through the core regression tests without a crash,
>> and with output diffs that seem fine to me. However, that just shows that
>> we lack enough test coverage; we evidently have no regression cases where
>> an upper node needs to print Vars that are coming from a fully-pruned
>> Append. Given the test case mentioned in this thread, I get
>> regression=# explain verbose select * from t1 where dt = current_date + 400;
>> ---------------------------------------------
>> Append (cost=0.00..198.42 rows=44 width=8)
>> Subplans Removed: 4
>> (2 rows)
>> which seems fine, but
>> regression=# explain verbose select * from t1 where dt = current_date + 400 order by id;
>> psql: server closed the connection unexpectedly
>> It's dying trying to resolve Vars in the Sort node, of course.
> Another approach, as I mentioned above, is to extend the hack that begins
> in nodeAppend.c (and nodeMergeAppend.c) into explain.c, as in the
> attached. Then:
> explain verbose select * from t1 where dt = current_date + 400 order by id;
> ───────────────────────────────────────────────────
> Sort (cost=199.62..199.73 rows=44 width=8)
> Output:, t1_1.dt
> Sort Key:
> -> Append (cost=0.00..198.42 rows=44 width=8)
> Subplans Removed: 4
> (5 rows)
> It's pretty confusing to see t1_1 which has been pruned away, but you
> didn't seem very interested in the idea of teaching explain.c to use the
> original target list of plans like Append, MergeAppend, etc. that have
> child subplans.
> Just a note: runtime pruning for MergeAppend is new in PG 12.

The patch I attached with the previous email didn't update the expected
output file. Correct one attached.


Attachment Content-Type Size
explain-fully-pruned-append-mergeappend-2.patch text/plain 7.7 KB

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