Tracking last scan time

Often it is beneficial to review one's schema with a view to removing
indexes (and sometimes tables) that are no longer required. It's very
difficult to understand when that is the case by looking at the number of
scans of a relation as, for example, an index may be used infrequently but
may be critical in those times when it is used.

The attached patch against HEAD adds optional tracking of the last scan
time for relations. It updates pg_stat_*_tables with new last_seq_scan and
last_idx_scan columns, and pg_stat_*_indexes with a last_idx_scan column to
help with this.

Due to the use of gettimeofday(), those values are only maintained if a new
GUC, track_scans, is set to on. By default, it is off.

I did run a 12 hour test to see what the performance impact is. pgbench was
run with scale factor 10000 and 75 users across 4 identical bare metal
machines running Rocky 8 in parallel which showed roughly a -2% average
performance penalty against HEAD with track_scans enabled. Machines were
PowerEdge R7525's with 128GB RAM, dual 16C/32T AMD 7302 CPUs, with the data
directory on 6 x 800GB 12Gb/s SSD SAS drives in RAID 0. Kernel time source
is tsc.

HEAD track_scans Penalty (%)
box1 19582.49735 19341.8881 -1.22869541
box2 19936.55513 19928.07479 -0.04253664659
box3 19631.78895 18649.64379 -5.002830696
box4 19810.86767 19420.67192 -1.969604525
Average 19740.42728 19335.06965 -2.05343896

Doc and test updates included.

--
Dave Page
Blog: https://pgsnake.blogspot.com
Twitter: @pgsnake

EDB: https://www.enterprisedb.com