On Tue, Jun 2, 2020 at 7:33 PM Justin Pryzby <pryzby@xxxxxxxxxxxxx> wrote:
> To: pgsql-general@xxxxxxxxxxxxxxxxxxxx, pgsql-performance@xxxxxxxxxxxxxxxxxxxx
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
I see. So it works with low cardinality in the partitioned column. With high cardinality an index scan on an unpartitioned table would be preferable I guess.
Regards,
-- Alex
