Date: Tue, 2 Jun 2020 12:33:54 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Oleksandr Shulgin <oleksandr.shulgin@zalando.de>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: When to use PARTITION BY HASH?
Message-ID: <20200602173353.GB30144@telsasoft.com>
References: 
 <CACACo5StuUuFox+65KQHB8u+51kMUcEeQuV=XmsUv7d8VnaO7A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CACACo5StuUuFox+65KQHB8u+51kMUcEeQuV=XmsUv7d8VnaO7A@mail.gmail.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Precedence: bulk

> To: pgsql-general@lists.postgresql.org, pgsql-performance@lists.postgresql.org

Please don't cross post to multiple lists.

On Tue, Jun 02, 2020 at 07:17:11PM +0200, Oleksandr Shulgin wrote:
> I was reading up on declarative partitioning[1] and I'm not sure what could
> be a possible application of Hash partitioning.

It's a good question.  See Tom's complaint here.
https://www.postgresql.org/message-id/31605.1586112900%40sss.pgh.pa.us

It *does* provide the benefit of smaller indexes and smaller tables, which
might allow seq scans to outpeform index scans.

It's maybe only useful for equality conditions on the partition key, and not
for ranges.  Here, it scans a single partition:

postgres=# CREATE TABLE t(i int) PARTITION BY HASH(i); CREATE TABLE t1 PARTITION OF t FOR VALUES WITH (REMAINDER 0, MODULUS 3);
postgres=# CREATE TABLE t2 PARTITION OF t FOR VALUES WITH (MODULUS 3, REMAINDER 1);
postgres=# CREATE TABLE t3 PARTITION OF t FOR VALUES WITH (MODULUS 3, REMAINDER 2);
postgres=# INSERT INTO t SELECT i%9 FROM generate_series(1,9999)i; ANALYZE t;
postgres=# explain analyze SELECT * FROM t WHERE i=3;
 Seq Scan on t2  (cost=0.00..75.55 rows=2222 width=4) (actual time=0.021..0.518 rows=2222 loops=1)
   Filter: (i = 3)
   Rows Removed by Filter: 2222

-- 
Justin