pgsql: Fix failure of btree_gin indexscans with "char" type and </<= op

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-committers(at)lists(dot)postgresql(dot)org
Subject: pgsql: Fix failure of btree_gin indexscans with "char" type and </<= op
Date: 2021-08-10 22:10:53
Message-ID: E1mDZxV-0002PI-K0@gemulon.postgresql.org
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Fix failure of btree_gin indexscans with "char" type and </<= operators.

As a result of confusion about whether the "char" type is signed or
unsigned, scans for index searches like "col < 'x'" or "col <= 'x'"
would start at the middle of the index not the left end, thus missing
many or all of the entries they should find. Fortunately, this
is not a symptom of index corruption. It's only the search logic
that is broken, and we can fix it without unpleasant side-effects.

Per report from Jason Kim. This has been wrong since btree_gin's
beginning, so back-patch to all supported branches.

Discussion: https://postgr.es/m/20210810001649.htnltbh7c63re42p@jasonk.me

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a4957b5a72dc9e169b78604f67357bb3dc49d826

Modified Files
--------------
contrib/btree_gin/btree_gin.c | 2 +-
contrib/btree_gin/expected/char.out | 11 +++++++++--
2 files changed, 10 insertions(+), 3 deletions(-)

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