Horton

Re: [POC] hash partitioning

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Cc: Jesper Pedersen <jesper.pedersen@redhat.com>, amul sul <sulamul@gmail.com>, Thom Brown <thom@linux.com>, Dilip Kumar <dilipbalaut@gmail.com>, Yugo Nagata <nagata@sraoss.co.jp>, Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>, David Steele <david@pgmasters.net>, Greg Stark <stark@mit.edu>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2017-09-29T16:53:39Z
Lists: pgsql-hackers 
On Thu, Sep 28, 2017 at 1:54 AM, Amit Langote
<Langote_Amit_f8@lab.ntt.co.jp> wrote:
> I looked into how satisfies_hash_partition() works and came up with an
> idea that I think will make constraint exclusion work.  What if we emitted
> the hash partition constraint in the following form instead:
>
> hash_partition_mod(hash_partition_hash(key1-exthash, key2-exthash),
>                    <mod>) = <rem>
>
> With that form, constraint exclusion seems to work as illustrated below:
>
> \d+ p0
> <...>
> Partition constraint:
> (hash_partition_modulus(hash_partition_hash(hashint4extended(a,
> '8816678312871386367'::bigint)), 4) = 0)
>
> -- note only p0 is scanned
> explain select * from p where
> hash_partition_modulus(hash_partition_hash(hashint4extended(a,
> '8816678312871386367'::bigint)), 4) = 0;

What we actually want constraint exclusion to cover is SELECT * FROM p
WHERE a = 525600;

As Amul says, nobody's going to enter a query in the form you have it
here.  Life is too short to take time to put queries into bizarre
forms.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Add hash partitioning.

  2. Add sanity check for pg_proc.provariadic

  3. Add hash_combine64.

  4. Make RelationGetPartitionDispatchInfo expand depth-first.

  5. Introduce 64-bit hash functions with a 64-bit seed.

  6. pg_dump: Add a --load-via-partition-root option.