pgsql: Avoid catalog lookups in RelationAllowsEarlyPruning().

Avoid catalog lookups in RelationAllowsEarlyPruning().

RelationAllowsEarlyPruning() performed a catalog scan, but is used
in two contexts where that was a bad idea:

1. In heap_page_prune_opt(), which runs very frequently in some large
scans. This caused major performance problems in a field report
that was easy to reproduce.

2. In TestForOldSnapshot(), which runs while we hold a buffer content
lock. It's not clear if this was guaranteed to be free of buffer
deadlock risk.

The check was introduced in commit 2cc41acd8 and defended against a
real problem: 9.6's hash indexes have no page LSN and so we can't
allow early pruning (ie the snapshot-too-old feature). We can remove
the check from all later releases though: hash indexes are now logged,
and there is no way to create UNLOGGED indexes on regular logged
tables.

If a future release allows such a combination, it might need to put
a similar check in place, but it'll need some more thought.

Back-patch to 10.

Author: Thomas Munro
Reviewed-by: Tom Lane, who spotted the second problem
Discussion: https://postgr.es/m/CA%2BhUKGKT8oTkp5jw_U4p0S-7UG9zsvtw_M47Y285bER6a2gD%2Bg%40mail.gmail.com
Discussion: https://postgr.es/m/CAA4eK1%2BWy%2BN4eE5zPm765h68LrkWc3Biu_8rzzi%2BOYX4j%2BiHRw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/720b59b55b84c8a346098cdbf3d34c7e554b0bf5

Modified Files
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src/backend/utils/cache/relcache.c | 42 --------------------------------------
src/include/utils/rel.h | 1 -
src/include/utils/snapmgr.h | 1 -
3 files changed, 44 deletions(-)