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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ROUTINE-VACUUMING"
>23.1. Routine Vacuuming</A
></H1
><A
NAME="AEN27860"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s
<A
HREF="sql-vacuum.html"
><I
>VACUUM</I
></A
> command has to run on
a regular basis for several reasons:
<P
></P
></P><OL
TYPE="1"
><LI
><P
>To recover or reuse disk space occupied by updated or deleted
rows.</P
></LI
><LI
><P
>To update data statistics used by the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> query planner.</P
></LI
><LI
><P
>To protect against loss of very old data due to
<I
CLASS="FIRSTTERM"
>transaction ID wraparound</I
>.</P
></LI
></OL
><P>
</P
><P
> The standard form of <TT
CLASS="COMMAND"
>VACUUM</TT
> can run in parallel with production
database operations. Commands such as <TT
CLASS="COMMAND"
>SELECT</TT
>,
<TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>, and <TT
CLASS="COMMAND"
>DELETE</TT
>
will continue to function as normal, though you will not be able to modify the
definition of a table with commands such as <TT
CLASS="COMMAND"
>ALTER TABLE ADD COLUMN</TT
>
while it is being vacuumed.
Also, <TT
CLASS="COMMAND"
>VACUUM</TT
> requires a substantial amount of I/O
traffic, which can cause poor performance for other active sessions.
There are configuration parameters that can be adjusted to reduce the
performance impact of background vacuuming — see
<A
HREF="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST"
>Section 18.4.4</A
>.
</P
><P
> Fortunately, <A
HREF="routine-vacuuming.html#AUTOVACUUM"
><I
>The Auto-Vacuum Daemon</I
></A
>
monitors table
activity and performs <TT
CLASS="COMMAND"
>VACUUM</TT
>s when necessary.
Autovacuum works dynamically so it is often better
administration-scheduled vacuuming.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-SPACE-RECOVERY"
>23.1.1. Recovering Disk Space</A
></H2
><A
NAME="AEN27888"
></A
><P
> In normal <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> operation, an
<TT
CLASS="COMMAND"
>UPDATE</TT
> or <TT
CLASS="COMMAND"
>DELETE</TT
> of a row does not
immediately remove the old version of the row.
This approach is necessary to gain the benefits of multiversion
concurrency control (see <A
HREF="mvcc.html"
>Chapter 13</A
>): the row versions
must not be deleted while it is still potentially visible to other
transactions. But eventually, an outdated or deleted row version is no
longer of interest to any transaction. The space it occupies must be
reclaimed for reuse by new rows, to avoid infinite growth of disk
space requirements. This is done by running <TT
CLASS="COMMAND"
>VACUUM</TT
>.
</P
><P
> There are two variants of the <TT
CLASS="COMMAND"
>VACUUM</TT
>
command. The first form, known as <SPAN
CLASS="QUOTE"
>"lazy vacuum"</SPAN
> or
just <TT
CLASS="COMMAND"
>VACUUM</TT
>, marks dead data in tables and
indexes for future reuse; it does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> attempt
to reclaim the space used by this dead data unless the space is
at the end of the table and an exclusive table lock can be easily
obtained. Unused space at the start or middle of the file does
not result in the file being shortened and space returned to the
operating system. This variant of <TT
CLASS="COMMAND"
>VACUUM</TT
> can be
run concurrently with normal database operations.
</P
><P
> The second form is the <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>
command. This uses a more aggressive algorithm for reclaiming the
space consumed by dead row versions. Any space that is freed by
<TT
CLASS="COMMAND"
>VACUUM FULL</TT
> is immediately returned to the
operating system, and the table data is physically compacted on
the disk. Unfortunately, this variant of the
<TT
CLASS="COMMAND"
>VACUUM</TT
> command acquires an exclusive lock on
each table while <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> is processing
it. Therefore, frequently using <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> can
have an extremely negative effect on the performance of concurrent
database queries.
</P
><P
> Fortunately, <A
HREF="routine-vacuuming.html#AUTOVACUUM"
><I
>The Auto-Vacuum Daemon</I
></A
>
monitors table
activity and performs <TT
CLASS="COMMAND"
>VACUUM</TT
>s when necessary. This
eliminates the need for administrators to worry about disk space
recovery in all but the most unusual cases.
</P
><P
> For administrators who want to control <TT
CLASS="COMMAND"
>VACUUM</TT
>
themselves, the standard form of <TT
CLASS="COMMAND"
>VACUUM</TT
> is best used to
maintain a steady-state usage of disk space. If you need to return
disk space to the operating system, you can use <TT
CLASS="COMMAND"
>VACUUM
FULL</TT
>, but this is unwise if the table will just grow again in the
future. Moderately-frequent standard <TT
CLASS="COMMAND"
>VACUUM</TT
> runs are a
better approach than infrequent <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> runs for
maintaining heavily-updated tables. However, if some heavily-updated
tables have gone too long with infrequent <TT
CLASS="COMMAND"
>VACUUM</TT
>, you can
use <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> or <TT
CLASS="COMMAND"
>CLUSTER</TT
> to get performance
back (it is much slower to scan a table containing almost only dead
rows).
</P
><P
> For those not using autovacuum, one approach is to schedule a
database-wide <TT
CLASS="COMMAND"
>VACUUM</TT
> once a day during low-usage period,
supplemented by more frequent vacuuming of heavily-updated tables if
necessary. (Some installations with extremely high update rates vacuum
their busiest tables as often as once every few minutes.) If you have
multiple databases in a cluster, don't forget to
<TT
CLASS="COMMAND"
>VACUUM</TT
> each one; the program <A
HREF="app-vacuumdb.html"
><I
><I
>vacuumdb</I
></I
></A
> might be helpful.
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> is recommended for cases where you know
you have deleted the majority of rows in a table, so that the
steady-state size of the table can be shrunk substantially with
<TT
CLASS="COMMAND"
>VACUUM FULL</TT
>'s more aggressive approach. Use plain
<TT
CLASS="COMMAND"
>VACUUM</TT
>, not <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>, for routine
vacuuming for space recovery.
</P
><P
> If you have a table whose entire contents are deleted on a periodic
basis, consider doing it with <TT
CLASS="COMMAND"
>TRUNCATE</TT
> rather
than using <TT
CLASS="COMMAND"
>DELETE</TT
> followed by
<TT
CLASS="COMMAND"
>VACUUM</TT
>. <TT
CLASS="COMMAND"
>TRUNCATE</TT
> removes the
entire content of the table immediately, without requiring a
subsequent <TT
CLASS="COMMAND"
>VACUUM</TT
> or <TT
CLASS="COMMAND"
>VACUUM
FULL</TT
> to reclaim the now-unused disk space.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-STATISTICS"
>23.1.2. Updating Planner Statistics</A
></H2
><A
NAME="AEN27938"
></A
><A
NAME="AEN27941"
></A
><P
> The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> query planner relies on
statistical information about the contents of tables in order to
generate good plans for queries. These statistics are gathered by
the <A
HREF="sql-analyze.html"
><I
>ANALYZE</I
></A
> command,
which can be invoked by itself or
as an optional step in <TT
CLASS="COMMAND"
>VACUUM</TT
>. It is important to have
reasonably accurate statistics, otherwise poor choices of plans might
degrade database performance.
</P
><P
> As with vacuuming for space recovery, frequent updates of statistics
are more useful for heavily-updated tables than for seldom-updated
ones. But even for a heavily-updated table, there might be no need for
statistics updates if the statistical distribution of the data is
not changing much. A simple rule of thumb is to think about how much
the minimum and maximum values of the columns in the table change.
For example, a <TT
CLASS="TYPE"
>timestamp</TT
> column that contains the time
of row update will have a constantly-increasing maximum value as
rows are added and updated; such a column will probably need more
frequent statistics updates than, say, a column containing URLs for
pages accessed on a website. The URL column might receive changes just
as often, but the statistical distribution of its values probably
changes relatively slowly.
</P
><P
> It is possible to run <TT
CLASS="COMMAND"
>ANALYZE</TT
> on specific tables and even
just specific columns of a table, so the flexibility exists to update some
statistics more frequently than others if your application requires it.
In practice, however, it is usually best to just analyze the entire database
because it is a fast operation. It uses a statistical random sampling of
the rows of a table rather than reading every single row.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Although per-column tweaking of <TT
CLASS="COMMAND"
>ANALYZE</TT
> frequency might not be
very productive, you might well find it worthwhile to do per-column
adjustment of the level of detail of the statistics collected by
<TT
CLASS="COMMAND"
>ANALYZE</TT
>. Columns that are heavily used in <TT
CLASS="LITERAL"
>WHERE</TT
> clauses
and have highly irregular data distributions might require a finer-grain
data histogram than other columns. See <TT
CLASS="COMMAND"
>ALTER TABLE SET
STATISTICS</TT
>.
</P
></BLOCKQUOTE
></DIV
><P
> Fortunately, <A
HREF="routine-vacuuming.html#AUTOVACUUM"
><I
>The Auto-Vacuum Daemon</I
></A
>
monitors table
activity and performs <TT
CLASS="COMMAND"
>ANALYZE</TT
>s when necessary. This
eliminates the need for administrators to manually schedule
<TT
CLASS="COMMAND"
>ANALYZE</TT
>.
</P
><P
> For those not using autovacuum, one approach is to schedule a
database-wide <TT
CLASS="COMMAND"
>ANALYZE</TT
> once a day at a low-usage time of
day; this can usefully be combined with a nightly <TT
CLASS="COMMAND"
>VACUUM</TT
>.
However, sites with relatively slowly changing table statistics might
find that this is overkill, and that less-frequent <TT
CLASS="COMMAND"
>ANALYZE</TT
>
runs are sufficient.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-WRAPAROUND"
>23.1.3. Preventing Transaction ID Wraparound Failures</A
></H2
><A
NAME="AEN27967"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s MVCC transaction semantics
depend on being able to compare transaction ID (<ACRONYM
CLASS="ACRONYM"
>XID</ACRONYM
>)
numbers: a row version with an insertion XID greater than the current
transaction's XID is <SPAN
CLASS="QUOTE"
>"in the future"</SPAN
> and should not be visible
to the current transaction. But since transaction IDs have limited size
(32 bits at this writing) a cluster that runs for a long time (more
than 4 billion transactions) would suffer <I
CLASS="FIRSTTERM"
>transaction ID
wraparound</I
>: the XID counter wraps around to zero, and all of a sudden
transactions that were in the past appear to be in the future — which
means their outputs become invisible. In short, catastrophic data loss.
(Actually the data is still there, but that's cold comfort if you cannot
get at it.) To avoid this, it is necessary to vacuum every table
in every database at least once every two billion transactions.
</P
><P
> The reason that periodic vacuuming solves the problem is that
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> distinguishes a special XID
<TT
CLASS="LITERAL"
>FrozenXID</TT
>. This XID is always considered older
than every normal XID. Normal XIDs are
compared using modulo-2<SUP
>31</SUP
> arithmetic. This means
that for every normal XID, there are two billion XIDs that are
<SPAN
CLASS="QUOTE"
>"older"</SPAN
> and two billion that are <SPAN
CLASS="QUOTE"
>"newer"</SPAN
>; another
way to say it is that the normal XID space is circular with no
endpoint. Therefore, once a row version has been created with a particular
normal XID, the row version will appear to be <SPAN
CLASS="QUOTE"
>"in the past"</SPAN
> for
the next two billion transactions, no matter which normal XID we are
talking about. If the row version still exists after more than two billion
transactions, it will suddenly appear to be in the future. To
prevent data loss, old row versions must be reassigned the XID
<TT
CLASS="LITERAL"
>FrozenXID</TT
> sometime before they reach the
two-billion-transactions-old mark. Once they are assigned this
special XID, they will appear to be <SPAN
CLASS="QUOTE"
>"in the past"</SPAN
> to all
normal transactions regardless of wraparound issues, and so such
row versions will be good until deleted, no matter how long that is.
This reassignment of old XIDs is handled by <TT
CLASS="COMMAND"
>VACUUM</TT
>.
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM</TT
>'s behavior is controlled by the configuration parameter
<A
HREF="runtime-config-client.html#GUC-VACUUM-FREEZE-MIN-AGE"
>vacuum_freeze_min_age</A
>: any XID older than
<TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
> transactions is replaced by
<TT
CLASS="LITERAL"
>FrozenXID</TT
>. Larger values of <TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
>
preserve transactional information longer, while smaller values increase
the number of transactions that can elapse before the table must be
vacuumed again.
</P
><P
> The maximum time that a table can go unvacuumed is two billion
transactions minus the <TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
> that was used
when it was last vacuumed.
If it were to go unvacuumed for longer than that,
data loss could result. To ensure that this does not
happen, <A
HREF="routine-vacuuming.html#AUTOVACUUM"
><I
>The Auto-Vacuum Daemon</I
></A
>
is invoked on any table
that might contain XIDs older than the age specified by the
configuration parameter
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-FREEZE-MAX-AGE"
>autovacuum_freeze_max_age</A
>. (This will happen
even if autovacuum is otherwise disabled.)
</P
><P
> This implies that if a table is not otherwise vacuumed,
autovacuum will be invoked on it approximately once every
<TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> minus
<TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
> transactions.
For tables that are regularly vacuumed for space reclamation purposes,
this is of little importance. However, for static tables
(including tables that receive inserts, but no updates or deletes),
there is no need for vacuuming for space reclamation, and so it can
be useful to try to maximize the interval between forced autovacuums
on very large static tables. Obviously one can do this either by
increasing <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> or by decreasing
<TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
>.
</P
><P
> The sole disadvantage of increasing <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
>
is that the <TT
CLASS="FILENAME"
>pg_clog</TT
> subdirectory of the database cluster
will take more space, because it must store the commit status for all
transactions back to the <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> horizon.
The commit status uses two bits per transaction, so if
<TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> has its maximum allowed value of
a little less than two billion, <TT
CLASS="FILENAME"
>pg_clog</TT
> can be expected to
grow to about half a gigabyte. If this is trivial compared to your
total database size, setting <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> to
its maximum allowed value is recommended. Otherwise, set it depending
on what you are willing to allow for <TT
CLASS="FILENAME"
>pg_clog</TT
> storage.
(The default, 200 million transactions, translates to about 50MB of
<TT
CLASS="FILENAME"
>pg_clog</TT
> storage.)
</P
><P
> One disadvantage of decreasing <TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
> is that
it might cause <TT
CLASS="COMMAND"
>VACUUM</TT
> to do useless work: changing a table row's
XID to <TT
CLASS="LITERAL"
>FrozenXID</TT
> is a waste of time if the row is modified
soon thereafter (causing it to acquire a new XID). So the setting should
be large enough that rows are not frozen until they are unlikely to change
any more. Another disadvantage of decreasing this setting is
that details about exactly which transaction inserted or modified a
row will be lost sooner. This information sometimes comes in handy,
particularly when trying to analyze what went wrong after a database
failure. For these two reasons, decreasing this setting is not
recommended except for completely static tables.
</P
><P
> To track the age of the oldest XIDs in a database,
<TT
CLASS="COMMAND"
>VACUUM</TT
> stores XID
statistics in the system tables <TT
CLASS="STRUCTNAME"
>pg_class</TT
> and
<TT
CLASS="STRUCTNAME"
>pg_database</TT
>. In particular,
the <TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
> column of a table's
<TT
CLASS="STRUCTNAME"
>pg_class</TT
> row contains the freeze cutoff XID that was used
by the last <TT
CLASS="COMMAND"
>VACUUM</TT
> for that table. All normal
XIDs older than this cutoff XID are guaranteed to have been replaced by
<TT
CLASS="LITERAL"
>FrozenXID</TT
> within the table. Similarly,
the <TT
CLASS="STRUCTFIELD"
>datfrozenxid</TT
> column of a database's
<TT
CLASS="STRUCTNAME"
>pg_database</TT
> row is a lower bound on the normal XIDs
appearing in that database — it is just the minimum of the
per-table <TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
> values within the database.
A convenient way to
examine this information is to execute queries such as:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT relname, age(relfrozenxid) FROM pg_class WHERE relkind = 'r';
SELECT datname, age(datfrozenxid) FROM pg_database;</PRE
><P>
The <TT
CLASS="LITERAL"
>age</TT
> column measures the number of transactions from the
cutoff XID to the current transaction's XID. Immediately after a
<TT
CLASS="COMMAND"
>VACUUM</TT
>, <TT
CLASS="LITERAL"
>age(relfrozenxid)</TT
> should be a little
more than the <TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
> setting that was used
(more by the number of transactions started since the <TT
CLASS="COMMAND"
>VACUUM</TT
>
started). If <TT
CLASS="LITERAL"
>age(relfrozenxid)</TT
> exceeds
<TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
>, an autovacuum will soon be forced
for the table.
</P
><P
> If for some reason autovacuum fails to clear old XIDs from a table,
the system will begin to emit warning messages like this when the
database's oldest XIDs reach ten million transactions from the wraparound
point:
</P><PRE
CLASS="PROGRAMLISTING"
>WARNING: database "mydb" must be vacuumed within 177009986 transactions
HINT: To avoid a database shutdown, execute a full-database VACUUM in "mydb".</PRE
><P>
If these warnings are
ignored, the system will shut down and refuse to execute any new
transactions once there are fewer than 1 million transactions left
until wraparound:
</P><PRE
CLASS="PROGRAMLISTING"
>ERROR: database is not accepting commands to avoid wraparound data loss in database "mydb"
HINT: Stop the postmaster and use a standalone backend to VACUUM in "mydb".</PRE
><P>
The 1-million-transaction safety margin exists to let the
administrator recover without data loss, by manually executing the
required <TT
CLASS="COMMAND"
>VACUUM</TT
> commands. However, since the system will not
execute commands once it has gone into the safety shutdown mode,
the only way to do this is to stop the server and use a single-user
backend to execute <TT
CLASS="COMMAND"
>VACUUM</TT
>. The shutdown mode is not enforced
by a single-user backend. See the <A
HREF="app-postgres.html"
><SPAN
CLASS="APPLICATION"
>postgres</SPAN
></A
> reference
page for details about using a single-user backend.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AUTOVACUUM"
>23.1.4. The Auto-Vacuum Daemon</A
></H2
><A
NAME="AEN28040"
></A
><P
> Beginning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.1, there is an
optional feature called <I
CLASS="FIRSTTERM"
>autovacuum</I
>,
whose purpose is to automate the execution of
<TT
CLASS="COMMAND"
>VACUUM</TT
> and <TT
CLASS="COMMAND"
>ANALYZE </TT
> commands.
When enabled, autovacuum checks for
tables that have had a large number of inserted, updated or deleted
tuples. These checks use the statistics collection facility;
therefore, autovacuum cannot be used unless <A
HREF="runtime-config-statistics.html#GUC-TRACK-COUNTS"
>track_counts</A
> is set to <TT
CLASS="LITERAL"
>true</TT
>.
In the default configuration, autovacuuming is enabled and the related
configuration parameters are appropriately set.
</P
><P
> Beginning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.3, autovacuum has a
multiprocess architecture: There is a daemon process, called the
<I
CLASS="FIRSTTERM"
>autovacuum launcher</I
>, which is in charge of starting
<I
CLASS="FIRSTTERM"
>autovacuum worker</I
> processes for all databases. The
launcher will distribute the work across time, but attempt to start one
worker on each database every <A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-NAPTIME"
>autovacuum_naptime</A
>
seconds. One worker will be launched for each database, with a maximum
of <A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
> processes running at the
same time. If there are more than
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
> databases to be processed,
the next database will be processed as soon as the first worker finishes.
The worker processes will check each table within its database and
execute <TT
CLASS="COMMAND"
>VACUUM</TT
> and/or <TT
CLASS="COMMAND"
>ANALYZE</TT
> as needed.
</P
><P
> The <A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
> setting limits how many
workers may be running at any time. If several large tables all become
eligible for vacuuming in a short amount of time, all autovacuum workers
may end up vacuuming those tables for a very long time. This would result
in other tables and databases not being vacuumed until a worker became
available. There is not a limit on how many workers might be in a
single database, but workers do try to avoid repeating work that has
already been done by other workers. Note that the number of running
workers does not count towards the <A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
> nor
the <A
HREF="runtime-config-connection.html#GUC-SUPERUSER-RESERVED-CONNECTIONS"
>superuser_reserved_connections</A
> limits.
</P
><P
> Tables whose <TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
> value is more than
<TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> transactions old are always
vacuumed. Otherwise,
two conditions are used to determine which operation(s)
to apply. If the number of obsolete tuples since the last
<TT
CLASS="COMMAND"
>VACUUM</TT
> exceeds the <SPAN
CLASS="QUOTE"
>"vacuum threshold"</SPAN
>, the
table is vacuumed. The vacuum threshold is defined as:
</P><PRE
CLASS="PROGRAMLISTING"
>vacuum threshold = vacuum base threshold + vacuum scale factor * number of tuples</PRE
><P>
where the vacuum base threshold is
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-THRESHOLD"
>autovacuum_vacuum_threshold</A
>,
the vacuum scale factor is
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-SCALE-FACTOR"
>autovacuum_vacuum_scale_factor</A
>,
and the number of tuples is
<TT
CLASS="STRUCTNAME"
>pg_class</TT
>.<TT
CLASS="STRUCTFIELD"
>reltuples</TT
>.
The number of obsolete tuples is obtained from the statistics
collector; it is a semi-accurate count updated by each
<TT
CLASS="COMMAND"
>UPDATE</TT
> and <TT
CLASS="COMMAND"
>DELETE</TT
> operation. (It
is only semi-accurate because some information might be lost under heavy
load.) For analyze, a similar condition is used: the threshold, defined as:
</P><PRE
CLASS="PROGRAMLISTING"
>analyze threshold = analyze base threshold + analyze scale factor * number of tuples</PRE
><P>
is compared to the total number of tuples inserted or updated
since the last <TT
CLASS="COMMAND"
>ANALYZE</TT
>.
</P
><P
> The default thresholds and scale factors are taken from
<TT
CLASS="FILENAME"
>postgresql.conf</TT
>, but it is possible to override them
on a table-by-table basis by making entries in the system catalog
<A
HREF="catalog-pg-autovacuum.html"
><TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
></A
>.
If a <TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
> row exists for a particular
table, the settings it specifies are applied; otherwise the global
settings are used. See <A
HREF="runtime-config-autovacuum.html"
>Section 18.9</A
> for
more details on the global settings.
</P
><P
> Besides the base threshold values and scale factors, there are five
more parameters that can be set for each table in
<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>.
The first, <TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>.<TT
CLASS="STRUCTFIELD"
>enabled</TT
>,
can be set to <TT
CLASS="LITERAL"
>false</TT
> to instruct the autovacuum daemon
to skip that particular table entirely. In this case
autovacuum will only touch the table if it must do so
to prevent transaction ID wraparound.
The next two parameters, the vacuum cost delay
(<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>.<TT
CLASS="STRUCTFIELD"
>vac_cost_delay</TT
>)
and the vacuum cost limit
(<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>.<TT
CLASS="STRUCTFIELD"
>vac_cost_limit</TT
>),
are used to set table-specific values for the
<A
HREF="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST"
><I
>Cost-Based Vacuum Delay</I
></A
>
feature.
The last two parameters,
(<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>.<TT
CLASS="STRUCTFIELD"
>freeze_min_age</TT
>)
and
(<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>.<TT
CLASS="STRUCTFIELD"
>freeze_max_age</TT
>),
are used to set table-specific values for
<A
HREF="runtime-config-client.html#GUC-VACUUM-FREEZE-MIN-AGE"
>vacuum_freeze_min_age</A
> and
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-FREEZE-MAX-AGE"
>autovacuum_freeze_max_age</A
> respectively.
</P
><P
> If any of the values in <TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
>
are set to a negative number, or if a row is not present at all in
<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
> for any particular table, the
corresponding values from <TT
CLASS="FILENAME"
>postgresql.conf</TT
> are used.
</P
><P
> There is not currently any support for making
<TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
> entries, except by doing
manual <TT
CLASS="COMMAND"
>INSERT</TT
>s into the catalog. This feature will be
improved in future releases, and it is likely that the catalog
definition will change.
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> The contents of the <TT
CLASS="STRUCTNAME"
>pg_autovacuum</TT
> system
catalog are currently not saved in database dumps created by the
tools <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> and <SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
>. If
you want to preserve them across a dump/reload cycle, make sure
you dump the catalog manually.
</P
></TD
></TR
></TABLE
></DIV
><P
> When multiple workers are running, the cost limit is
<SPAN
CLASS="QUOTE"
>"balanced"</SPAN
> among all the running workers, so that the
total impact on the system is the same, regardless of the number
of workers actually running.
</P
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