row-level stats and last analyze time

[Neil Conway <neilc(at)samurai(dot)com>]

[ CC'ing -hackers ]

On Sun, 2007-04-22 at 16:10 +0200, Guillaume Lelarge wrote:
> This patch adds a sentence on monitoring.sgml explaining that 
> stats_row_level needs to be enabled if user wants to get last 
> vacuum/analyze execution time.

This behavior was introduced in r1.120 of postmaster/pgstat.c:

    Modify pgstats code to reduce performance penalties from
    oversized stats data files: avoid creating stats
    hashtable entries for tables that aren't being touched
    except by vacuum/analyze [...]

which included other modifications to reduce the pgstat I/O volume in
8.1. I don't think this particular change was wise: the reduction in
pgstat volume is pretty marginal, and it is counter-intuitive for
stats_row_level to effect whether the last ANALYZE / VACUUM is recorded.
(Plus, the optimization is not even enabled with the default
postgresql.conf settings.)

-Neil