Re: vacuuming problems continued - Mailing list pgsql-performance

Andrew Sullivan wrote:
> On Thu, Jun 01, 2006 at 01:54:08PM +0200, Antoine wrote:
>> Hi,
>> We just don't seem to be getting much benefit from autovacuum. Running
>> a manual vacuum seems to still be doing a LOT, which suggests to me
>> that I should either run a cron job and disable autovacuum, or just
>> run a cron job on top of autovacuum.

Don't know if this was covered in an earlier thread. Bear with me if so.

I'm working with 7.4.8 and 8.0.3 systems, and pg_autovacuum does have some
glitches ... in part solved by the integrated autovac in 8.1:

- in our env, clients occasionally hit max_connections. This is a known and
(sort of) desired pushback on load. However, that sometimes knocks pg_autovacuum
out.

- db server goes down for any reason: same problem.


Just restarting pg_autovacuum is not good enough; when pg_autovacuum terminates,
it loses its state, so big tables that change less than 50% between such
terminations may never get vacuumed (!)

For that reason, it's taken a switch to a Perl script run from cron every 5
minutes, that persists state in a table. The script is not a plug-compatible
match for pg_autovacuum (hardcoded rates; hardcoded distinction between user and
system tables), but you may find it useful.

--
Engineers think that equations approximate reality.
Physicists think that reality approximates the equations.
Mathematicians never make the connection.
#!/usr/bin/env perl

use strict;
use warnings;
use Carp;
use DBI;
use POSIX        qw(strftime);

# Hardcoded (aggressive) autovacuum parameters:
my ($VAC_BASE, $VAC_RATE)  = (1000, 0.8);
my ($ANA_BASE, $ANA_RATE)  = ( 500, 0.4);

my $VERBOSE = 'VERBOSE';
my $start = time;
my $stamp = strftime "==== %FT%T autovac: ", localtime;

open STDERR, ">&STDOUT"; # Redirect PG "VERBOSE" output.
my $dbh = DBI->connect("dbi:Pg(PrintError=1,RaiseError=1):");

# REVISIT: move this to schema:

my $oid = $dbh->selectall_arrayref(<<"__SQL__")->[0][0];
    SELECT oid FROM pg_class WHERE relname = 'autovac_state';
__SQL__

$dbh->do(<<"__SQL__") if !defined $oid;
    CREATE TABLE public.autovac_state(
        relid           oid NOT NULL PRIMARY KEY,
        name            text NOT NULL,
        analyze_time    timestamptz,
        vacuum_time     timestamptz,
        analyze_tups bigint, -- (n_tup_ins+n_tup_upd+n_tup_del) at last ANALYZE
        vacuum_tups  bigint  -- (n_tup_upd+n_tup_del) at last VACUUM
    );
__SQL__

# Calculate updates/additions to autovac_state:

my $anavac = join ";", map {$_->[0]} @{$dbh->selectall_arrayref(<<"__SQL__")};

    SELECT * INTO TEMP new_state FROM (
    SELECT
        relid,      -- identify tables by ID, so that (re)created tables always
                --      are treated as fresh tables.
                name,       -- for constructing the vacuum/analyze command
        old_relid,  -- NULL means this will need a new state table entry
        analyze_tups,-- _tups are used to update autovac_state
        vacuum_tups,

        CASE WHEN analyze_tups - prev_analyze_tups NOT BETWEEN 0 AND analyze_point
            OR old_relid IS NULL
            THEN now()
        END AS analyze_time,

        CASE WHEN vacuum_tups - prev_vacuum_tups NOT BETWEEN 0 AND vacuum_point
            THEN now()
        END AS vacuum_time

    FROM (
        SELECT  N.nspname || '.' || C.relname               AS name,
            A.relid                                     AS old_relid,
            C.oid                                       AS relid,
            S.n_tup_ins + S.n_tup_upd + S.n_tup_del     AS analyze_tups,
            S.n_tup_upd + S.n_tup_del                   AS vacuum_tups,
            COALESCE(A.analyze_tups,0)                   AS prev_analyze_tups,
            COALESCE(A.vacuum_tups,0)                    AS prev_vacuum_tups,
            CASE WHEN nspname ~ '^pg_' THEN 1.0 ELSE $ANA_RATE END
            * C.reltuples + $ANA_BASE        AS analyze_point,
            CASE WHEN nspname ~ '^pg_' THEN 2.0 ELSE $VAC_RATE END
            * C.reltuples + $VAC_BASE        AS vacuum_point
        FROM    pg_class                AS C
        JOIN    pg_namespace            AS N ON N.oid = C.relnamespace
        JOIN    pg_stat_all_tables      AS S ON S.relid = C.oid
        LEFT JOIN autovac_state         AS A ON A.relid = S.relid
        WHERE   N.nspname NOT LIKE 'pg_temp%'
    ) AS X
    ) AS X
    WHERE   analyze_time IS NOT NULL OR vacuum_time IS NOT NULL;

    SELECT CASE WHEN vacuum_time IS NOT NULL
                THEN 'VACUUM ANALYZE $VERBOSE ' || name
                ELSE 'ANALYZE $VERBOSE ' || name
            END
    FROM new_state;
__SQL__

if ($anavac) {
    print STDERR $stamp."start\n";

    $dbh->do(<<"__SQL__");

        $anavac;

        UPDATE  autovac_state
        SET     analyze_tups = N.analyze_tups,
                vacuum_tups     = CASE WHEN N.vacuum_time IS NULL
                                       THEN autovac_state.vacuum_tups
                                       ELSE N.vacuum_tups
                                  END,
                analyze_time    = COALESCE(N.analyze_time, autovac_state.analyze_time),
                vacuum_time     = COALESCE(N.vacuum_time, autovac_state.vacuum_time)
        FROM    new_state AS N
        WHERE   N.relid = autovac_state.relid;

        INSERT INTO autovac_state
        SELECT  relid, name, analyze_time, vacuum_time, analyze_tups, vacuum_tups
        FROM    new_state
    WHERE     old_relid IS NULL;

    DELETE FROM autovac_state
    WHERE    analyze_time < now() - '1 day'::INTERVAL
    AND    relid NOT IN (SELECT oid FROM pg_class);
__SQL__

    print STDERR $stamp.(time - $start)." secs\n";
}

$dbh->do("DROP TABLE new_state");

1;
__END__

=head1 SYNOPSIS

autovac - autovacuum with persistent state.

=head1 DESCRIPTION

C<autovac> is a replacement for C<pg_autovacuum>.
C<autovac> does a single C<pg_autovacuum> step, then saves its state in
the C<public.autovac_state> table. It should be run from cron, say,
every 5 minutes.


C<autovac> runs more aggressively for user tables
(pg_autovacuum -V 0.8 -v 1000) than for system tables.

When pg_stats_user_tables.(ins/upd/del) counts have been zeroed,
C<autovac> vacuums all tables.

To check when C<autovac> last analyzed/vacuumed a given table:

    SELECT vacuum_time, analyze_time FROM autovac_state
    WHERE name = 'public.message'

=head1 OUTPUT

If any action is taken, C<autovac> prints:

    ==== yyyy-mm-dd hh:mm:ss autovac start
    ... VACUUM/ANALYZE VERBOSE output ...
    ==== yyyy-mm-dd hh:mm:ss autovac <n> secs

=head1 PG_AUTOVACUUM

=cut