pgsql: Make type Datum be 8 bytes wide everywhere.

Make type Datum be 8 bytes wide everywhere.

This patch makes sizeof(Datum) be 8 on all platforms including
32-bit ones.  The objective is to allow USE_FLOAT8_BYVAL to be true
everywhere, and in consequence to remove a lot of code that is
specific to pass-by-reference handling of float8, int8, etc.  The
code for abbreviated sort keys can be simplified similarly.  In this
way we can reduce the maintenance effort involved in supporting 32-bit
platforms, without going so far as to actually desupport them.  Since
Datum is strictly an in-memory concept, this has no impact on on-disk
storage, though an initdb or pg_upgrade will be needed to fix affected
catalog entries.

We have required platforms to support [u]int64 for ages, so this
breaks no supported platform.  We can expect that this change will
make 32-bit builds a bit slower and more memory-hungry, although being
able to use pass-by-value handling of 8-byte types may buy back some
of that.  But we stopped optimizing for 32-bit cases a long time ago,
and this seems like just another step on that path.

This initial patch simply forces the correct type definition and
USE_FLOAT8_BYVAL setting, and cleans up a couple of minor compiler
complaints that ensued.  This is sufficient for testing purposes.
In the wake of a bunch of Datum-conversion cleanups by Peter
Eisentraut, this now compiles cleanly with gcc on a 32-bit platform.
(I'd only tested the previous version with clang, which it turns out
is less picky than gcc about width-changing coercions.)

There is a good deal of now-dead code that I'll remove in separate
follow-up patches.

A catversion bump is required because this affects initial catalog
contents (on 32-bit machines) in two ways: pg_type.typbyval changes
for some built-in types, and Const nodes in stored views/rules will
now have 8 bytes not 4 for pass-by-value types.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/1749799.1752797397@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2a600a93c7be5b0bf8cacb1af78009db12bc4857

Modified Files
--------------
src/backend/storage/ipc/ipc.c         |  2 +-
src/backend/utils/resowner/resowner.c |  7 ++-----
src/include/catalog/catversion.h      |  2 +-
src/include/nodes/nodes.h             |  8 ++++++--
src/include/pg_config_manual.h        | 13 ++++---------
src/include/postgres.h                | 21 +++++++++++++--------
6 files changed, 27 insertions(+), 26 deletions(-)