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Add empty BRIN ranges during CREATE INDEX

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When building BRIN indexes, the brinbuildCallback only advances to the
next page range when seeing a tuple that doesn't belong to the current
one. This means that the index may end up missing ranges at the end of
the table, if those pages do not contain any indexable tuples.

We tend not to have completely empty pages at the end of a relation, but
this also applies to partial indexes, where the tuples may simply not
match the index predicate. This results in inefficient scans using the
affected BRIN index - without the summaries, the page ranges have to be
read and processed, which consumes I/O and possibly also CPU time.

The existing code already added empty ranges for earlier parts of the
table, this commit makes sure we add them for the ranges at the end of
the table too.

Patch by Matthias van de Meent, with review/improvements by me.

Author: Matthias van de Meent
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/CAEze2WiMsPZg%3DxkvSF_jt4%3D69k6K7gz5B8V2wY3gCGZ%2B1BzCbQ%40mail.gmail.com
This commit is contained in:
Tomas Vondra
2023-12-08 17:07:30 +01:00
parent 00edb2061f
commit dae761a87e
3 changed files with 152 additions and 4 deletions
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17
contrib/pageinspect/sql/brin.sql
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@ -36,4 +36,21 @@ SELECT brin_page_items(decode(repeat('00', :block_size), 'hex'), 'test1_a_idx');
SELECT brin_metapage_info(decode(repeat('00', :block_size), 'hex'));
SELECT brin_revmap_data(decode(repeat('00', :block_size), 'hex'));
-- Test that partial indexes have all pages, including empty ones.
CREATE TABLE test2 (a int);
INSERT INTO test2 SELECT i FROM generate_series(1,1000) s(i);
-- No rows match the index predicate, make sure the index has the right number
-- of ranges (same as number of page ranges).
CREATE INDEX ON test2 USING brin (a) WITH (pages_per_range=1) WHERE (a IS NULL);
ANALYZE test2;
-- Does the index have one summary of the relation?
SELECT (COUNT(*) = (SELECT relpages FROM pg_class WHERE relname = 'test2')) AS ranges_do_match
FROM generate_series((SELECT (lastrevmappage + 1) FROM brin_metapage_info(get_raw_page('test2_a_idx', 0))),
(SELECT (relpages - 1) FROM pg_class WHERE relname = 'test2_a_idx')) AS pages(p),
LATERAL brin_page_items(get_raw_page('test2_a_idx', p), 'test2_a_idx') AS items;
DROP TABLE test1;
DROP TABLE test2;