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><DIV
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><H1
><A
NAME="MAINTENANCE"
></A
>Chapter 21. Routine Database Maintenance Tasks</H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>21.1. <A
HREF="maintenance.html#ROUTINE-VACUUMING"
>Routine Vacuuming</A
></DT
><DD
><DL
><DT
>21.1.1. <A
HREF="maintenance.html#VACUUM-FOR-SPACE-RECOVERY"
>Recovering disk space</A
></DT
><DT
>21.1.2. <A
HREF="maintenance.html#VACUUM-FOR-STATISTICS"
>Updating planner statistics</A
></DT
><DT
>21.1.3. <A
HREF="maintenance.html#VACUUM-FOR-WRAPAROUND"
>Preventing transaction ID wraparound failures</A
></DT
></DL
></DD
><DT
>21.2. <A
HREF="routine-reindex.html"
>Routine Reindexing</A
></DT
><DT
>21.3. <A
HREF="logfile-maintenance.html"
>Log File Maintenance</A
></DT
></DL
></DIV
><A
NAME="AEN21035"
></A
><P
> There are a few routine maintenance chores that must be performed on
a regular basis to keep a <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
server running smoothly. The tasks discussed here are repetitive
in nature and can easily be automated using standard Unix tools such
as <SPAN
CLASS="APPLICATION"
>cron</SPAN
> scripts. But it is the database
administrator's responsibility to set up appropriate scripts, and to
check that they execute successfully.
</P
><P
> One obvious maintenance task is creation of backup copies of the data on a
regular schedule. Without a recent backup, you have no chance of recovery
after a catastrophe (disk failure, fire, mistakenly dropping a critical
table, etc.). The backup and recovery mechanisms available in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> are discussed at length in
<A
HREF="backup.html"
>Chapter 22</A
>.
</P
><P
> The other main category of maintenance task is periodic <SPAN
CLASS="QUOTE"
>"vacuuming"</SPAN
>
of the database. This activity is discussed in
<A
HREF="maintenance.html#ROUTINE-VACUUMING"
>Section 21.1</A
>.
</P
><P
> Something else that might need periodic attention is log file management.
This is discussed in <A
HREF="logfile-maintenance.html"
>Section 21.3</A
>.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is low-maintenance compared
to some other database management systems. Nonetheless,
appropriate attention to these tasks will go far towards ensuring a
pleasant and productive experience with the system.
</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ROUTINE-VACUUMING"
>21.1. Routine Vacuuming</A
></H1
><A
NAME="AEN21052"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s <TT
CLASS="COMMAND"
>VACUUM</TT
> command
must be run on a regular basis for several reasons:
<P
></P
></P><OL
TYPE="1"
><LI
><P
>To recover 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>
The frequency and scope of the <TT
CLASS="COMMAND"
>VACUUM</TT
> operations performed for each of
these reasons will vary depending on the needs of each site.
Therefore, database administrators must understand these issues and
develop an appropriate maintenance strategy. This section concentrates
on explaining the high-level issues; for details about command syntax
and so on, see the <A
HREF="sql-vacuum.html"
>VACUUM</A
> reference page.
</P
><P
> Beginning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.2, the standard form
of <TT
CLASS="COMMAND"
>VACUUM</TT
> can run in parallel with normal database operations
(selects, inserts, updates, deletes, but not changes to table definitions).
Routine vacuuming is therefore not nearly as intrusive as it was in prior
releases, and it is not as critical to try to schedule it at low-usage
times of day.
</P
><P
> Beginning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.0, there are
configuration parameters that can be adjusted to further reduce the
performance impact of background vacuuming. See
<A
HREF="runtime-config.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST"
>Section 16.4.3.4</A
>.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="VACUUM-FOR-SPACE-RECOVERY"
>21.1.1. Recovering disk space</A
></H2
><A
NAME="AEN21076"
></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 12</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 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
> Clearly, a table that receives frequent updates or deletes will need
to be vacuumed more often than tables that are seldom updated. It
may be useful to set up periodic <SPAN
CLASS="APPLICATION"
>cron</SPAN
> tasks that
<TT
CLASS="COMMAND"
>VACUUM</TT
> only selected tables, skipping tables that are known not to
change often. This is only likely to be helpful if you have both
large heavily-updated tables and large seldom-updated tables — the
extra cost of vacuuming a small table isn't enough to be worth
worrying about.
</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 expired 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 expired data
immediately. Therefore, the table file is not shortened, and any
unused space in the file is not 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 expired row versions. Any space that is freed by
<TT
CLASS="COMMAND"
>VACUUM FULL</TT
> is immediately returned to the
operating system. 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
> The standard form of <TT
CLASS="COMMAND"
>VACUUM</TT
> is best used with the goal
of maintaining a fairly level 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 what's the point of releasing disk
space that will only have to be allocated again soon? 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
> Recommended practice for most sites is to schedule a database-wide
<TT
CLASS="COMMAND"
>VACUUM</TT
> once a day at a low-usage time of day,
supplemented by more frequent vacuuming of heavily-updated tables
if necessary. (Some installations with an extremely high
rate of data modification <TT
CLASS="COMMAND"
>VACUUM</TT
> busy 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 <TT
CLASS="FILENAME"
>vacuumdb</TT
> may be helpful.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> The <TT
CLASS="FILENAME"
>contrib/pg_autovacuum</TT
> program can be useful for
automating high-frequency vacuuming operations.
</P
></BLOCKQUOTE
></DIV
><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 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"
>21.1.2. Updating planner statistics</A
></H2
><A
NAME="AEN21126"
></A
><A
NAME="AEN21129"
></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 <TT
CLASS="COMMAND"
>ANALYZE</TT
> 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 may
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 may 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 may 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, the usefulness of this feature is doubtful.
Beginning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.2,
<TT
CLASS="COMMAND"
>ANALYZE</TT
> is a fairly fast operation even on large tables,
because it uses a statistical random sampling of the rows of a table
rather than reading every single row. So it's probably much simpler
to just run it over the whole database every so often.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Although per-column tweaking of <TT
CLASS="COMMAND"
>ANALYZE</TT
> frequency may not be
very productive, you may 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 may require a finer-grain
data histogram than other columns. See <TT
CLASS="COMMAND"
>ALTER TABLE SET
STATISTICS</TT
>.
</P
></BLOCKQUOTE
></DIV
><P
> Recommended practice for most sites 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 may 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"
>21.1.3. Preventing transaction ID wraparound failures</A
></H2
><A
NAME="AEN21153"
></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) will 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 can't
get at it.)
</P
><P
> Prior to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 7.2, the only defense
against XID wraparound was to re-<TT
CLASS="COMMAND"
>initdb</TT
> at least every 4
billion transactions. This of course was not very satisfactory for
high-traffic sites, so a better solution has been devised. The new
approach allows a server to remain up indefinitely, without
<TT
CLASS="COMMAND"
>initdb</TT
> or any sort of restart. The price is this
maintenance requirement: <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>every table in the database must
be vacuumed at least once every billion transactions</I
></SPAN
>.
</P
><P
> In practice this isn't an onerous requirement, but since the
consequences of failing to meet it can be complete data loss (not
just wasted disk space or slow performance), some special provisions
have been made to help database administrators keep track of the
time since the last <TT
CLASS="COMMAND"
>VACUUM</TT
>. The remainder of this
section gives the details.
</P
><P
> The new approach to XID comparison distinguishes two special XIDs,
numbers 1 and 2 (<TT
CLASS="LITERAL"
>BootstrapXID</TT
> and
<TT
CLASS="LITERAL"
>FrozenXID</TT
>). These two XIDs are always considered older
than every normal XID. Normal XIDs (those greater than 2) 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 XID is handled by <TT
CLASS="COMMAND"
>VACUUM</TT
>.
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM</TT
>'s normal policy is to reassign <TT
CLASS="LITERAL"
>FrozenXID</TT
>
to any row version with a normal XID more than one billion transactions in the
past. This policy preserves the original insertion XID until it is not
likely to be of interest anymore. (In fact, most row versions will probably
live and die without ever being <SPAN
CLASS="QUOTE"
>"frozen"</SPAN
>.) With this policy,
the maximum safe interval between <TT
CLASS="COMMAND"
>VACUUM</TT
> runs on any table
is exactly one billion transactions: if you wait longer, it's possible
that a row version that was not quite old enough to be reassigned last time
is now more than two billion transactions old and has wrapped around
into the future — i.e., is lost to you. (Of course, it'll reappear
after another two billion transactions, but that's no help.)
</P
><P
> Since periodic <TT
CLASS="COMMAND"
>VACUUM</TT
> runs are needed anyway for the reasons
described earlier, it's unlikely that any table would not be vacuumed
for as long as a billion transactions. But to help administrators ensure
this constraint is met, <TT
CLASS="COMMAND"
>VACUUM</TT
> stores transaction ID
statistics in the system table <TT
CLASS="LITERAL"
>pg_database</TT
>. In particular,
the <TT
CLASS="LITERAL"
>datfrozenxid</TT
> column of a database's
<TT
CLASS="LITERAL"
>pg_database</TT
> row is updated at the completion of any
database-wide <TT
CLASS="COMMAND"
>VACUUM</TT
> operation (i.e., <TT
CLASS="COMMAND"
>VACUUM</TT
> that does not
name a specific table). The value stored in this field is the freeze
cutoff XID that was used by that <TT
CLASS="COMMAND"
>VACUUM</TT
> command. All normal
XIDs older than this cutoff XID are guaranteed to have been replaced by
<TT
CLASS="LITERAL"
>FrozenXID</TT
> within that database. A convenient way to
examine this information is to execute the query
</P><PRE
CLASS="PROGRAMLISTING"
>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
> With the standard freezing policy, the <TT
CLASS="LITERAL"
>age</TT
> column will start
at one billion for a freshly-vacuumed database. When the <TT
CLASS="LITERAL"
>age</TT
>
approaches two billion, the database must be vacuumed again to avoid
risk of wraparound failures. Recommended practice is to <TT
CLASS="COMMAND"
>VACUUM</TT
> each
database at least once every half-a-billion (500 million) transactions,
so as to provide plenty of safety margin. To help meet this rule,
each database-wide <TT
CLASS="COMMAND"
>VACUUM</TT
> automatically delivers a warning
if there are any <TT
CLASS="LITERAL"
>pg_database</TT
> entries showing an
<TT
CLASS="LITERAL"
>age</TT
> of more than 1.5 billion transactions, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>play=# VACUUM;
WARNING: some databases have not been vacuumed in 1613770184 transactions
HINT: Better vacuum them within 533713463 transactions, or you may have a wraparound failure.
VACUUM</PRE
><P>
</P
><P
> <TT
CLASS="COMMAND"
>VACUUM</TT
> with the <TT
CLASS="COMMAND"
>FREEZE</TT
> option uses a more
aggressive freezing policy: row versions are frozen if they are old enough
to be considered good by all open transactions. In particular, if a
<TT
CLASS="COMMAND"
>VACUUM FREEZE</TT
> is performed in an otherwise-idle
database, it is guaranteed that <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all</I
></SPAN
> row versions in that
database will be frozen. Hence, as long as the database is not
modified in any way, it will not need subsequent vacuuming to avoid
transaction ID wraparound problems. This technique is used by
<TT
CLASS="COMMAND"
>initdb</TT
> to prepare the <TT
CLASS="LITERAL"
>template0</TT
> database.
It should also be used to prepare any user-created databases that
are to be marked <TT
CLASS="LITERAL"
>datallowconn</TT
> = <TT
CLASS="LITERAL"
>false</TT
> in
<TT
CLASS="LITERAL"
>pg_database</TT
>, since there isn't any convenient way to
<TT
CLASS="COMMAND"
>VACUUM</TT
> a database that you can't connect to. Note that
<TT
CLASS="COMMAND"
>VACUUM</TT
>'s automatic warning message about
unvacuumed databases will ignore <TT
CLASS="LITERAL"
>pg_database</TT
> entries
with <TT
CLASS="LITERAL"
>datallowconn</TT
> = <TT
CLASS="LITERAL"
>false</TT
>, so as to avoid
giving false warnings about these databases; therefore it's up to
you to ensure that such databases are frozen correctly.
</P
><DIV
CLASS="WARNING"
><P
></P
><TABLE
CLASS="WARNING"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Warning</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> To be sure of safety against transaction wraparound, it is necessary
to vacuum <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>every</I
></SPAN
> table, including system catalogs, in
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>every</I
></SPAN
> database at least once every billion transactions.
We have seen data loss situations caused by people deciding that they
only needed to vacuum their active user tables, rather than issuing
database-wide vacuum commands. That will appear to work fine ...
for a while.
</P
></TD
></TR
></TABLE
></DIV
></DIV
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
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