From: Jim Finnerty <jfinnert@amazon.com>
To: pgsql-hackers@postgresql.org
Subject: Re: insensitive collations
Date: Thu, 25 Mar 2021 06:09:49 -0700 (MST)
Message-ID: <1616677789675-0.post@n3.nabble.com> (raw)
In-Reply-To: <54ef5bfe-043e-4328-9d70-5818789838f4@manitou-mail.org>
References: <cdd0ed2e-1d98-6fc4-07f1-23f19f53b201@2ndquadrant.com>
<5c7f51b0-6788-4ffd-b9c4-23b710373140@manitou-mail.org>
<fd54ad77-fb1a-c77d-1371-d7850562b080@2ndquadrant.com>
<440ec733-d068-46d3-9756-f5f692aba7c8@manitou-mail.org>
<99818c53-d10f-eb0b-bcb1-fd5d60f6dd96@2ndquadrant.com>
<f2b3b8e8-12c4-ceb7-6803-b4af6eb2cb71@2ndquadrant.com>
<693ad06d-db9a-4e59-8131-f823483c5893@manitou-mail.org>
<1616528764665-0.post@n3.nabble.com>
<1616618498263-0.post@n3.nabble.com>
<54ef5bfe-043e-4328-9d70-5818789838f4@manitou-mail.org>
The two ideas can be combined to create a workaround for accent-sensitive
nondeterministic collations that enables an ordinary btree to be exploited
if available, and also provides the full LIKE logic in either case-sensitive
or case-insensitive collations:
SELECT * FROM locations WHERE location LIKE 'midi-Pyrén%ées';
becomes:
SELECT * FROM locations
WHERE
location COLLATE "C" ILIKE 'midi-Pyrén%ées' AND
-- For CI collations only
location BETWEEN 'midi-Pyrén' AND 'midi-Pyrén' || E'\uFFFF'; --
exploitable by ordinary btree
LIKE would also be valid for a case sensitive collation, but then the
collation would be CS_AS, so it would be deterministic and no transform
would be needed.
The expression above produces a good plan, but EXPLAIN complains if the
concatenated expression is not a valid character for the current
client_encoding, which I had set to WIN1252 to display the accented
characters properly on the client:
babel=# SELECT * FROM locations
babel-# WHERE
babel-# location COLLATE "C" ILIKE 'midi-Pyrén%ées' AND
-- For CI collations only
babel-# location BETWEEN 'midi-Pyrén' AND 'midi-Pyrén' || E'\uFFFF';
-- exploitable by ordinary btree
location
-----------------
Midi-Pyrénées
midi-Pyrénées
(2 rows)
babel=# EXPLAIN VERBOSE SELECT * FROM locations
babel-# WHERE
babel-# location COLLATE "C" ILIKE 'midi-Pyrén%ées' AND
-- For CI collations only
babel-# location BETWEEN 'midi-Pyrén' AND 'midi-Pyrén' || E'\uFFFF';
-- exploitable by ordinary btree
ERROR: character with byte sequence 0xef 0xbf 0xbf in encoding "UTF8" has
no equivalent in encoding "WIN1252"
reset client_encoding;
babel=# EXPLAIN VERBOSE SELECT * FROM locations
WHERE
location COLLATE "C" ILIKE 'midi-Pyrén%ées' AND
-- For CI collations only
location BETWEEN 'midi-Pyrén' AND 'midi-Pyrén' || E'\uFFFF'; --
exploitable by ordinary btree
QUERY PLAN
------------------------------------------------------------------------------------------------------------
Index Only Scan using *location_index* on public.locations
(cost=0.13..8.15 rows=1 width=18)
Output: location
Index Cond: ((locations.location >= 'midi-Pyrén'::text) AND
(locations.location <= 'midi-Pyrén'::text))
Filter: ((locations.location)::text ~~* 'midi-Pyrén%ées'::text)
(4 rows)
-----
Jim Finnerty, AWS, Amazon Aurora PostgreSQL
--
Sent from: https://www.postgresql-archive.org/PostgreSQL-hackers-f1928748.html
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To: pgsql-hackers@postgresql.org
Cc: jfinnert@amazon.com
Subject: Re: insensitive collations
In-Reply-To: <1616677789675-0.post@n3.nabble.com>
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