Re: [POC] hash partitioning

On Thu, Sep 14, 2017 at 11:39 AM, Jesper Pedersen
<jesper(dot)pedersen(at)redhat(dot)com> wrote:
> When I do
>
> CREATE TABLE mytab (
> a integer NOT NULL,
> b integer NOT NULL,
> c integer,
> d integer
> ) PARTITION BY HASH (b);
>
> and create 64 partitions;
>
> CREATE TABLE mytab_p00 PARTITION OF mytab FOR VALUES WITH (MODULUS 64,
> REMAINDER 0);
> ...
> CREATE TABLE mytab_p63 PARTITION OF mytab FOR VALUES WITH (MODULUS 64,
> REMAINDER 63);
>
> and associated indexes
>
> CREATE INDEX idx_p00 ON mytab_p00 USING btree (b, a);
> ...
> CREATE INDEX idx_p63 ON mytab_p63 USING btree (b, a);
>
> Populate the database, and do ANALYZE.
>
> Given
>
> EXPLAIN (ANALYZE, VERBOSE, BUFFERS ON) SELECT a, b, c, d FROM mytab WHERE b
> = 42
>
> gives
>
> Append
> -> Index Scan using idx_p00 (cost rows=7) (actual rows=0)
> ...
> -> Index Scan using idx_p63 (cost rows=7) (actual rows=0)
>
> E.g. all partitions are being scanned. Of course one partition will contain
> the rows I'm looking for.

Yeah, we need Amit Langote's work in
http://postgr.es/m/098b9c71-1915-1a2a-8d52-1a7a50ce79e8@lab.ntt.co.jp
to land and this patch to be adapted to make use of it. I think
that's the major thing still standing in the way of this. Concerns
were also raised about not having a way to see the hash function, but
we fixed that in 81c5e46c490e2426db243eada186995da5bb0ba7 and
hopefully this patch has been updated to use a seed (I haven't looked
yet). And there was a concern about hash functions not being
portable, but the conclusion of that was basically that most people
think --load-via-partition-root will be a satisfactory workaround for
cases where that becomes a problem (cf. commit
23d7680d04b958de327be96ffdde8f024140d50e). So this is the major
remaining issue that I know about.

--
Robert Haas
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The Enterprise PostgreSQL Company