pgsql: Fix dynahash's HASH_FIXED_SIZE ("isfixed") option.

Fix dynahash's HASH_FIXED_SIZE ("isfixed") option.

This flag was effectively a no-op in EXEC_BACKEND (ie, Windows)
builds, because it was kept in the process-local HTAB struct,
and it could only ever become set in the postmaster's copy.

The simplest fix is to move it to the shared HASHHDR struct.
We could keep a copy in HTAB as well, as we do with keysize
and some other fields, but the "too much contention" argument
doesn't seem to apply here: we only examine isfixed during
element_alloc(), which had better not get hit very often for
a shared hashtable.

This oversight dates to 7c797e719 which invented the option.
But back-patching doesn't seem appropriate given the lack of
field complaints. If there is anyone running an affected
workload on Windows, they might be unhappy about the behavior
changing in a minor release.

Author: Aidar Imamov <a(dot)imamov(at)postgrespro(dot)ru>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/4d0cb35ff01c5c74d2b9a582ecb73823@postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5457ea46d181f8b8dbe1ae482720b23bff4029de

Modified Files
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src/backend/utils/hash/dynahash.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)