Horton

Re: PROC_IN_ANALYZE stillborn 13 years ago

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, James Coleman <jtc331@gmail.com>, pgsql-hackers <pgsql-hackers@lists.postgresql.org>, Simon Riggs <simon@2ndquadrant.com>
Date: 2020-08-06T22:02:26Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:
> In fact using conceptually like a new snapshot for each sample tuple
> actually seems like it'd be somewhat of an improvement over using a
> single snapshot.

Dunno, that feels like a fairly bad idea to me.  It seems like it would
overemphasize the behavior of whatever queries happened to be running
concurrently with the ANALYZE.  I do follow the argument that using a
single snapshot for the whole ANALYZE overemphasizes a single instant
in time, but I don't think that leads to the conclusion that we shouldn't
use a snapshot at all.

Another angle that would be worth considering, aside from the issue
of whether the sample used for pg_statistic becomes more or less
representative, is what impact all this would have on the tuple count
estimates that go to the stats collector and pg_class.reltuples.
Right now, we don't have a great story at all on how the stats collector's
count is affected by combining VACUUM/ANALYZE table-wide counts with
the incremental deltas reported by transactions happening concurrently
with VACUUM/ANALYZE.  Would changing this behavior make that better,
or worse, or about the same?

			regards, tom lane

Commits

  1. Call out vacuum considerations in create index docs

  2. Document concurrent indexes waiting on each other

  3. snapshot scalability: Don't compute global horizons while building snapshots.

  4. Remove PROC_IN_ANALYZE and derived flags

  5. Improve performance of get_actual_variable_range with recently-dead tuples.