Re: Problem with default partition pruning

From: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
To: Thibaut Madelaine <thibaut(dot)madelaine(at)dalibo(dot)com>, Yuzuko Hosoya <hosoya(dot)yuzuko(at)lab(dot)ntt(dot)co(dot)jp>, "'Imai, Yoshikazu'" <imai(dot)yoshikazu(at)jp(dot)fujitsu(dot)com>
Cc: "'PostgreSQL Hackers'" <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Problem with default partition pruning
Date: 2019-03-20 09:06:13
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Hi Thibaut,

On 2019/03/19 23:58, Thibaut Madelaine wrote:
> I kept on testing with sub-partitioning.

> I found a case, using 2 default partitions, where a default partition is
> not pruned:
> --------------
> create table test2(id int, val text) partition by range (id);
> create table test2_20_plus_def partition of test2 default;
> create table test2_0_20 partition of test2 for values from (0) to (20)
>   partition by range (id);
> create table test2_0_10 partition of test2_0_20 for values from (0) to (10);
> create table test2_10_20_def partition of test2_0_20 default;
> # explain (costs off) select * from test2 where id=5 or id=25;
>                QUERY PLAN               
> -----------------------------------------
>  Append
>    ->  Seq Scan on test2_0_10
>          Filter: ((id = 5) OR (id = 25))
>    ->  Seq Scan on test2_10_20_def
>          Filter: ((id = 5) OR (id = 25))
>    ->  Seq Scan on test2_20_plus_def
>          Filter: ((id = 5) OR (id = 25))
> (7 rows)
> --------------
> I have the same output using Amit's v1-delta.patch or Hosoya's
> v2_default_partition_pruning.patch.

I think I've figured what may be wrong.

Partition pruning step generation code should ignore any arguments of an
OR clause that won't be true for a sub-partitioned partition, given its
partition constraint.

In this case, id = 25 contradicts test2_0_20's partition constraint (which
is, a IS NOT NULL AND a >= 0 AND a < 20), so the OR clause should really
be simplified to id = 5, ignoring the id = 25 argument. Note that we
remove id = 25 only for the considerations of pruning and not from the
actual clause that's passed to the final plan, although it wouldn't be a
bad idea to try to do that.

Attached revised delta patch, which includes the fix described above.


Attachment Content-Type Size
v1-delta.patch text/plain 3.2 KB

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