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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ROUTINE-VACUUMING"
>23.1. Routine Vacuuming</A
></H1
><A
NAME="AEN30188"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> databases require periodic
maintenance known as <I
CLASS="FIRSTTERM"
>vacuuming</I
>. For many installations, it
is sufficient to let vacuuming be performed by the <I
CLASS="FIRSTTERM"
>autovacuum
daemon</I
>, which is described in <A
HREF="routine-vacuuming.html#AUTOVACUUM"
>Section 23.1.5</A
>. You might
need to adjust the autovacuuming parameters described there to obtain best
results for your situation. Some database administrators will want to
supplement or replace the daemon's activities with manually-managed
<TT
CLASS="COMMAND"
>VACUUM</TT
> commands, which typically are executed according to a
schedule by <SPAN
CLASS="APPLICATION"
>cron</SPAN
> or <SPAN
CLASS="APPLICATION"
>Task
Scheduler</SPAN
> scripts. To set up manually-managed vacuuming properly,
it is essential to understand the issues discussed in the next few
subsections. Administrators who rely on autovacuuming may still wish
to skim this material to help them understand and adjust autovacuuming.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-BASICS"
>23.1.1. Vacuuming Basics</A
></H2
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s
<A
HREF="sql-vacuum.html"
><I
>VACUUM</I
></A
> command has to
process each table 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>
Each of these reasons dictates performing <TT
CLASS="COMMAND"
>VACUUM</TT
> operations
of varying frequency and scope, as explained in the following subsections.
</P
><P
> There are two variants of <TT
CLASS="COMMAND"
>VACUUM</TT
>: standard <TT
CLASS="COMMAND"
>VACUUM</TT
>
and <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>. <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> can reclaim more
disk space but runs much more slowly. Also,
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</TT
> while it is being vacuumed.)
<TT
CLASS="COMMAND"
>VACUUM FULL</TT
> requires exclusive lock on the table it is
working on, and therefore cannot be done in parallel with other use
of the table. Another disadvantage of <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> is that
while it reduces table size, it does not reduce index size proportionally;
in fact it can make indexes <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>larger</I
></SPAN
>. Generally, therefore,
administrators should strive to use standard <TT
CLASS="COMMAND"
>VACUUM</TT
> and
avoid <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>.
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM</TT
> creates 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 18.4.3</A
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-SPACE-RECOVERY"
>23.1.2. Recovering Disk Space</A
></H2
><A
NAME="AEN30234"
></A
><P
> In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, 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 version
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 then 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
> The standard form of <TT
CLASS="COMMAND"
>VACUUM</TT
> removes dead row
versions in tables and indexes and marks the space available for
future reuse. However, it will not return the space to the operating
system, except in the special case where one or more pages at the
end of a table become entirely free and an exclusive table lock can be
easily obtained. In contrast, <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> actively compacts
tables by moving row versions to earlier pages. It is thus able to
force pages at the end of the table to become entirely free, whereupon
it will return them to the operating system. However, if many rows
must be moved, this can take a long time. Also, moving a row requires
transiently making duplicate index entries for it (the entry pointing
to its new location must be made before the old entry can be removed);
so moving a lot of rows this way causes severe index bloat.
</P
><P
> The usual goal of routine vacuuming is to do standard <TT
CLASS="COMMAND"
>VACUUM</TT
>s
often enough to avoid needing <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>. The
autovacuum daemon attempts to work this way, and in fact will
never issue <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>. In this approach, the idea
is not to keep tables at their minimum size, but to maintain steady-state
usage of disk space: each table occupies space equivalent to its
minimum size plus however much space gets used up between vacuumings.
Although <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> can be used to shrink a table back
to its minimum size and return the disk space to the operating system,
there is not much point in this if the table will just grow again in the
future. Thus, 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.
</P
><P
> Some administrators prefer to schedule vacuuming themselves, for example
doing all the work at night when load is low.
The difficulty with doing vacuuming according to a fixed schedule
is that if a table has an unexpected spike in update activity, it may
get bloated to the point that <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> is really necessary
to reclaim space. Using the autovacuum daemon alleviates this problem,
since the daemon schedules vacuuming dynamically in response to update
activity. It is unwise to disable the daemon completely unless you
have an extremely predictable workload. One possible compromise is
to set the daemon's parameters so that it will only react to unusually
heavy update activity, thus keeping things from getting out of hand,
while scheduled <TT
CLASS="COMMAND"
>VACUUM</TT
>s are expected to do the bulk of the
work when the load is typical.
</P
><P
> For those not using autovacuum, a typical approach is to schedule a
database-wide <TT
CLASS="COMMAND"
>VACUUM</TT
> once a day during a low-usage period,
supplemented by more frequent vacuuming of heavily-updated tables as
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
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Neither form of <TT
CLASS="COMMAND"
>VACUUM</TT
> is entirely satisfactory when
a table contains large numbers of dead row versions as a result of
massive update or delete activity. If you have such a table and
you need to reclaim the excess disk space it occupies, the best
way is to use <A
HREF="sql-cluster.html"
><I
>CLUSTER</I
></A
>
or one of the table-rewriting variants of
<A
HREF="sql-altertable.html"
><I
>ALTER TABLE</I
></A
>.
These commands rewrite an entire new copy of the table and build
new indexes for it. Like <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>, they require
exclusive lock. Note that they also temporarily use extra disk
space, since the old copies of the table and indexes can't be
released until the new ones are complete. In the worst case where
your disk is nearly full, <TT
CLASS="COMMAND"
>VACUUM FULL</TT
> may be the only
workable alternative.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> If you have a table whose entire contents are deleted on a periodic
basis, consider doing it with
<A
HREF="sql-truncate.html"
><I
>TRUNCATE</I
></A
> 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.
The disadvantage is that strict MVCC semantics are violated.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-STATISTICS"
>23.1.3. Updating Planner Statistics</A
></H2
><A
NAME="AEN30276"
></A
><A
NAME="AEN30279"
></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
> The autovacuum daemon, if enabled, will automatically issue
<TT
CLASS="COMMAND"
>ANALYZE</TT
> commands whenever the content of a table has
changed sufficiently. However, administrators might prefer to rely
on manually-scheduled <TT
CLASS="COMMAND"
>ANALYZE</TT
> operations, particularly
if it is known that update activity on a table will not affect the
statistics of <SPAN
CLASS="QUOTE"
>"interesting"</SPAN
> columns. The daemon schedules
<TT
CLASS="COMMAND"
>ANALYZE</TT
> strictly as a function of the number of rows
inserted or updated; it has no knowledge of whether that will lead
to meaningful statistical changes.
</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. <TT
CLASS="COMMAND"
>ANALYZE</TT
> 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
>, or change the database-wide default using the <A
HREF="runtime-config-query.html#GUC-DEFAULT-STATISTICS-TARGET"
>default_statistics_target</A
> configuration parameter.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-WRAPAROUND"
>23.1.4. Preventing Transaction ID Wraparound Failures</A
></H2
><A
NAME="AEN30304"
></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
> <A
HREF="runtime-config-client.html#GUC-VACUUM-FREEZE-MIN-AGE"
>vacuum_freeze_min_age</A
>
controls how old an XID value has to be before it's replaced with
<TT
CLASS="LITERAL"
>FrozenXID</TT
>. Larger values of this setting
preserve transactional information longer, while smaller values increase
the number of transactions that can elapse before the table must be
vacuumed again.
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM</TT
> normally skips pages that don't have any dead row
versions, but those pages might still have row versions with old XID
values. To ensure all old XIDs have been replaced by
<TT
CLASS="LITERAL"
>FrozenXID</TT
>, a scan of the whole table is needed.
<A
HREF="runtime-config-client.html#GUC-VACUUM-FREEZE-TABLE-AGE"
>vacuum_freeze_table_age</A
> controls when
<TT
CLASS="COMMAND"
>VACUUM</TT
> does that: a whole table sweep is forced if
the table hasn't been fully scanned for <TT
CLASS="VARNAME"
>vacuum_freeze_table_age</TT
>
minus <TT
CLASS="VARNAME"
>vacuum_freeze_min_age</TT
> transactions. Setting it to 0
forces <TT
CLASS="COMMAND"
>VACUUM</TT
> to always scan all pages, effectively ignoring
the visibility map.
</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 <TT
CLASS="COMMAND"
>VACUUM</TT
> last scanned the whole table. If it were to go
unvacuumed for longer than
that, data loss could result. To ensure that this does not happen,
autovacuum 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 effective maximum for <TT
CLASS="VARNAME"
>vacuum_freeze_table_age</TT
> is 0.95 *
<TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
>; a setting higher than that will be
capped to the maximum. A value higher than
<TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> wouldn't make sense because an
anti-wraparound autovacuum would be triggered at that point anyway, and
the 0.95 multiplier leaves some breathing room to run a manual
<TT
CLASS="COMMAND"
>VACUUM</TT
> before that happens. As a rule of thumb,
<TT
CLASS="COMMAND"
>vacuum_freeze_table_age</TT
> should be set to a value somewhat
below <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
>, leaving enough gap so that
a regularly scheduled <TT
CLASS="COMMAND"
>VACUUM</TT
> or an autovacuum triggered by
normal delete and update activity is run in that window. Setting it too
close could lead to anti-wraparound autovacuums, even though the table
was recently vacuumed to reclaim space, whereas lower values lead to more
frequent whole-table scans.
</P
><P
> The sole disadvantage of increasing <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
>
(and <TT
CLASS="VARNAME"
>vacuum_freeze_table_age</TT
> along with it)
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 whole-table <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.
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM</TT
> normally
only scans pages that have been modified since the last vacuum, but
<TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
> can only be advanced when the whole table is
scanned. The whole table is scanned when <TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
> is
more than <TT
CLASS="VARNAME"
>vacuum_freeze_table_age</TT
> transactions old, when the
<TT
CLASS="COMMAND"
>VACUUM FREEZE</TT
> command is used, or when all pages happen to
require vacuuming to remove dead row versions. When <TT
CLASS="COMMAND"
>VACUUM</TT
>
scans the whole table, after it's finished <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 no whole-table-scanning <TT
CLASS="COMMAND"
>VACUUM</TT
>
is issued on the table until <TT
CLASS="VARNAME"
>autovacuum_freeze_max_age</TT
> is
reached, 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 database-wide VACUUM in "mydb".</PRE
><P>
(A manual <TT
CLASS="COMMAND"
>VACUUM</TT
> should fix the problem, as suggested by the
hint; but note that the <TT
CLASS="COMMAND"
>VACUUM</TT
> must be performed by a
superuser, else it will fail to process system catalogs and thus not
be able to advance the database's <TT
CLASS="STRUCTFIELD"
>datfrozenxid</TT
>.)
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"
>23.1.5. The Autovacuum Daemon</A
></H2
><A
NAME="AEN30400"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> has an optional but highly
recommended 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
> The <SPAN
CLASS="QUOTE"
>"autovacuum daemon"</SPAN
> actually consists of multiple processes.
There is a persistent 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, attempting 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.
Each worker process 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 become occupied with vacuuming those tables for a long period.
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
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-FREEZE-MAX-AGE"
>autovacuum_freeze_max_age</A
> transactions old are always
vacuumed (this also applies to those tables whose freeze max age has
been modified via storage parameters; see below). Otherwise, if the
number of tuples obsoleted 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.) If the <TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
> value of the table is more
than <TT
CLASS="VARNAME"
>vacuum_freeze_table_age</TT
> transactions old, the whole
table is scanned to freeze old tuples and advance
<TT
CLASS="STRUCTFIELD"
>relfrozenxid</TT
>, otherwise only pages that have been modified
since the last vacuum are scanned.
</P
><P
> 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; see
<A
HREF="sql-createtable.html#SQL-CREATETABLE-STORAGE-PARAMETERS"
><I
>Storage Parameters</I
></A
> for more information.
If a setting
has been changed via storage parameters, that value is used; 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 six
more autovacuum parameters that can be set for each table via
storage parameters.
The first parameter, <TT
CLASS="LITERAL"
>autovacuum_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.
Another two parameters,
<TT
CLASS="LITERAL"
>autovacuum_vacuum_cost_delay</TT
> and
<TT
CLASS="LITERAL"
>autovacuum_vacuum_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.
<TT
CLASS="LITERAL"
>autovacuum_freeze_min_age</TT
>,
<TT
CLASS="LITERAL"
>autovacuum_freeze_max_age</TT
> and
<TT
CLASS="LITERAL"
>autovacuum_freeze_table_age</TT
> are used to set
values for <A
HREF="runtime-config-client.html#GUC-VACUUM-FREEZE-MIN-AGE"
>vacuum_freeze_min_age</A
>,
<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-FREEZE-MAX-AGE"
>autovacuum_freeze_max_age</A
> and
<A
HREF="runtime-config-client.html#GUC-VACUUM-FREEZE-TABLE-AGE"
>vacuum_freeze_table_age</A
> respectively.
</P
><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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