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 *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.
The documentation page I've linked only contains examples around partitioning BY RANGE. I believe it'd be helpful to extend it with some meaningful examples for LIST and HASH partitioning.
