Subject: Re: WIP: BRIN multi-range indexes
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: John Naylor <john.naylor@enterprisedb.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
References: <20210327180913.GA3088@alvherre.pgsql>
From: Tomas Vondra <tomas.vondra@enterprisedb.com>
Message-ID: <f673a88a-5d68-da56-8da8-08c2a4eff610@enterprisedb.com>
Date: Sat, 27 Mar 2021 21:42:05 +0100
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On 3/27/21 7:09 PM, Alvaro Herrera wrote:
> On 2021-Mar-26, Tomas Vondra wrote:
> 
>> Hi,
>>
>> I've pushed both the bloom and minmax-multi indexes today.
> 
> One thing I've been wondering all along is how useful are these
> BRIN-backed bloom indexes compared to contrib-supplied bloom indexes.
> My guess is that the BRIN implementation has some advantage, or you
> would not have worked so much on it.  But what is it?
> 

The contrib/bloom indexes are a completely different type of index. They
are not BRIN but a completely separate AM. The bloom filters are per-row
(so the index is larger than BRIN) and it's useful when you have table
with many attributes, and need to test various combinations of them.


create table t (a int, b int, c int);

insert into t select 10 * random(), 10 * random(), 10 * random()
  from generate_series(1,1000000) s(i);

analyze t;

create index bloom_idx on t using bloom (a,b,c)
  with (length=80, col1=4, col2=4, col3=4);

create index brin_bloom_idx on t using
  brin (a int4_bloom_ops, b int4_bloom_ops, c int4_bloom_ops);


test=# \di+
                          List of relations
 Schema |      Name      | Table | Access Method | Size  | Description
--------+----------------+-------+---------------+-------+-------------
 public | bloom_idx      | t     | bloom         | 15 MB |
 public | brin_bloom_idx | t     | brin          | 88 kB |
(2 rows)


So it's a completely different kind of animal, perhaps closer to btree
than to BRIN. I'm sure there are cases where contrib/bloom works better
than brin/bloom, but also the other way around.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
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