From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Greg Stark <stark@mit.edu>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Zhihong Yu <zyu@yugabyte.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: PATCH: Using BRIN indexes for sorted output
Date: Sat, 18 Feb 2023 12:51:09 -0600
Message-ID: <20230218185109.GH1653@telsasoft.com> (raw)
In-Reply-To: <8f1374d1-6f4b-ce99-84eb-ffa852c0dadb@enterprisedb.com>
References: <CAEze2Wi7S7bMeUpMyBbnGLrFgcWQTDN1edSXYcd8WixeBj9B_w@mail.gmail.com>
<87964cda-e440-8759-2838-bc4628072a18@enterprisedb.com>
<CAM-w4HMirdJ-c4chCZhni_T=9R65z2Zm3Ajg5mBrQ9s6OufZXQ@mail.gmail.com>
<14568731-ac91-ce2d-1477-0e2504c72775@enterprisedb.com>
<20221206185143.ivy3afwmkhloksgv@awork3.anarazel.de>
<4e679562-fd9a-33a3-ce71-cd7a27525f8e@enterprisedb.com>
<e0b6e395-3ab7-6021-b058-3c5901912b36@enterprisedb.com>
<20230216161016.GV1653@telsasoft.com>
<9f73f798-1dac-c75d-d834-aa012ba13b65@enterprisedb.com>
<8f1374d1-6f4b-ce99-84eb-ffa852c0dadb@enterprisedb.com>
Are (any of) these patches targetting v16 ?
typos:
ar we - we are?
morestly - mostly
interstect - intersect
> + * XXX We don't sort the bins, so just do binary sort. For large number of values
> + * this might be an issue, for small number of values a linear search is fine.
"binary sort" is wrong?
> + * only half of there ranges, thus 1/2. This can be extended to randomly
half of *these* ranges ?
> From 7b3307c27b35ece119feab4891f03749250e454b Mon Sep 17 00:00:00 2001
> From: Tomas Vondra <tomas.vondra@postgresql.org>
> Date: Mon, 17 Oct 2022 18:39:28 +0200
> Subject: [PATCH 01/11] Allow index AMs to build and use custom statistics
I think the idea can also apply to btree - currently, correlation is
considered to be a property of a column, but not an index. But that
fails to distinguish between a freshly built index, and an index with
out of order heap references, which can cause an index scan to be a lot
more expensive.
I implemented per-index correlation stats way back when:
https://www.postgresql.org/message-id/flat/20160524173914.GA11880%40telsasoft.com
See also:
https://www.postgresql.org/message-id/14438.1512499811@sss.pgh.pa.us
With my old test case:
Index scan is 3x slower than bitmap scan, but index scan is costed as
being cheaper:
postgres=# explain analyze SELECT * FROM t WHERE i>11 AND i<55;
Index Scan using t_i_idx on t (cost=0.43..21153.74 rows=130912 width=8) (actual time=0.107..222.737 rows=128914 loops=1)
postgres=# SET enable_indexscan =no;
postgres=# explain analyze SELECT * FROM t WHERE i>11 AND i<55;
Bitmap Heap Scan on t (cost=2834.28..26895.96 rows=130912 width=8) (actual time=16.830..69.860 rows=128914 loops=1)
If it's clustered, then the index scan is almost twice as fast, and the
costs are more consistent with the associated time. The planner assumes
that the indexes are freshly built...
postgres=# CLUSTER t USING t_i_idx ;
postgres=# explain analyze SELECT * FROM t WHERE i>11 AND i<55;
Index Scan using t_i_idx on t (cost=0.43..20121.74 rows=130912 width=8) (actual time=0.084..117.549 rows=128914 loops=1)
--
Justin
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Cc: pryzby@telsasoft.com, tomas.vondra@enterprisedb.com, andres@anarazel.de, stark@mit.edu, boekewurm+postgres@gmail.com, zyu@yugabyte.com, pgsql-hackers@lists.postgresql.org
Subject: Re: PATCH: Using BRIN indexes for sorted output
In-Reply-To: <20230218185109.GH1653@telsasoft.com>
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