After autovacuum collects the relations to vacuum or analyze, it rechecks
whether each relation still needs to be vacuumed or analyzed before actually
doing that. Previously this recheck could be a significant overhead
especially when there were a very large number of relations. This was
because each recheck forced the statistics to be refreshed, and the refresh
of the statistics for a very large number of relations could cause heavy
overhead. There was the report that this issue caused autovacuum workers
to have gotten “stuck” in a tight loop of table_recheck_autovac() that
rechecks whether a relation needs to be vacuumed or analyzed.

This commit speeds up the recheck by making autovacuum worker reuse
the previously-read statistics for the recheck if possible. Then if that
"stale" statistics says that a relation still needs to be vacuumed or analyzed,
autovacuum refreshes the statistics and does the recheck again.

The benchmark shows that the more relations exist and autovacuum workers
are running concurrently, the more this change reduces the autovacuum
execution time. For example, when there are 20,000 tables and 10 autovacuum
workers are running, the benchmark showed that the change improved
the performance of autovacuum more than three times. On the other hand,
even when there are only 1000 tables and only a single autovacuum worker
is running, the benchmark didn't show any big performance regression by
the change.

Firstly POC patch was proposed by Jim Nasby. As the result of discussion,
we used Tatsuhito Kasahara's version of the patch using the approach
suggested by Tom Lane.

Reported-by: Jim Nasby
Author: Tatsuhito Kasahara
Reviewed-by: Masahiko Sawada, Fujii Masao
Discussion: https://postgr.es/m/3FC6C2F2-8A47-44C0-B997-28830B5716D0@amazon.com