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Subject: BUG #19587: hashchar (internal "char" type) depends on platform char
 signedness
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: malis@pgrust.com
Reply-To: malis@pgrust.com, pgsql-bugs@lists.postgresql.org
Date: Wed, 29 Jul 2026 17:55:45 +0000
Message-ID: <19587-4416a590531c9c73@postgresql.org>
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 <https://www.postgresql.org/message-id/19587-4416a590531c9c73%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19587
Logged by:          Michael Malis
Email address:      malis@pgrust.com
PostgreSQL version: 18.4
Operating system:   Linux x86_64 and Linux aarch64, same package build
Description:       =20

This is about the internal single-byte "char" type (pg_type oid 18, the
one needing double quotes), not SQL CHAR(n), which is bpchar and is
unaffected.

hashchar() and hashcharextended() cast through a bare char, whose
signedness is implementation-defined:

hashchar(PG_FUNCTION_ARGS)
{ return hash_uint32((int32) PG_GETARG_CHAR(0)); }

DatumGetChar() returns char, so a high-bit byte sign-extends where
char is signed (x86-64, macOS) and does not where it is unsigned
(Linux aarch64).  Other integer hash functions cast from explicitly-signed
types and are unaffected.

Reproduce (SQL_ASCII database, on each platform):

SELECT hashchar(chr(128)::"char");

  x86-64:   1361043915      (=3D hashint4(-128))
  aarch64:  1807103465      (=3D hashint4( 128))

Impact. Hash aggregation and hash joins are unaffected, since a query
hashes both sides with the same binary.  The problem is where the hash is
persisted: hash partitioning on a "char" column routes rows differently
per platform, so a pg_dump taken on one architecture fails to restore on
the other (violates partition constraint; --load-via-partition-root
avoids it), and a hash index on such a column returns false negatives if
the data directory is read by a build with the opposite signedness.