Table of Contents
There are a few routine maintenance chores that must be performed on a regular basis to keep a PostgreSQL server running smoothly. The tasks discussed here are repetitive in nature and can easily be automated using standard Unix tools such as cron scripts. But it is the database administrator's responsibility to set up appropriate scripts, and to check that they execute successfully.
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 PostgreSQL are discussed at length in Chapter 23, Backup and Restore.
The other main category of maintenance task is periodic “vacuuming” of the database. This activity is discussed in Section 22.1, “Routine Vacuuming”.
Something else that might need periodic attention is log file management. This is discussed in Section 22.3, “Log File Maintenance”.
PostgreSQL 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.
PostgreSQL's VACUUM
command
must be run on a regular basis for several reasons:
- To recover disk space occupied by updated or deleted rows.
- To update data statistics used by the PostgreSQL query planner.
- To protect against loss of very old data due to transaction ID wraparound.
The frequency and scope of the VACUUM
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 VACUUM reference page.
Beginning in PostgreSQL 7.2, the standard form
of VACUUM
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.
Beginning in PostgreSQL 8.0, there are configuration parameters that can be adjusted to further reduce the performance impact of background vacuuming. See Section 17.4.4, “ Cost-Based Vacuum Delay ”.
An automated mechanism for performing the necessary VACUUM
operations has been added in PostgreSQL 8.1.
See Section 22.1.4, “The auto-vacuum daemon”.
In normal PostgreSQL operation, an
UPDATE
or DELETE
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 Chapter 12, Concurrency Control): 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 VACUUM
.
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 cron tasks that
VACUUM
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 [mdash ] the
extra cost of vacuuming a small table isn't enough to be worth
worrying about.
There are two variants of the VACUUM
command. The first form, known as “lazy vacuum” or
just VACUUM
, marks expired data in tables and
indexes for future reuse; it does not 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 VACUUM
can be run
concurrently with normal database operations.
The second form is the VACUUM FULL
command. This uses a more aggressive algorithm for reclaiming the
space consumed by expired row versions. Any space that is freed by
VACUUM FULL
is immediately returned to the
operating system. Unfortunately, this variant of the
VACUUM
command acquires an exclusive lock on
each table while VACUUM FULL
is processing
it. Therefore, frequently using VACUUM FULL
can
have an extremely negative effect on the performance of concurrent
database queries.
The standard form of VACUUM
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
VACUUM FULL
[mdash ] but what's the point of releasing disk
space that will only have to be allocated again soon? Moderately
frequent standard VACUUM
runs are a better approach
than infrequent VACUUM FULL
runs for maintaining
heavily-updated tables.
Recommended practice for most sites is to schedule a database-wide
VACUUM
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 VACUUM
busy tables as
often as once every few minutes.) If you have multiple databases
in a cluster, don't forget to VACUUM
each one;
the program vacuumdb
may be helpful.
VACUUM FULL
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
VACUUM FULL
's more aggressive approach. Use plain
VACUUM
, not VACUUM FULL
, for routine
vacuuming for space recovery.
If you have a table whose contents are deleted on a periodic
basis, consider doing it with TRUNCATE
rather
than using DELETE
followed by
VACUUM
. TRUNCATE
removes the
entire content of the table immediately, without requiring a
subsequent VACUUM
or VACUUM
FULL
to reclaim the now-unused disk space.
The PostgreSQL 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 ANALYZE
command, which can be invoked by itself or
as an optional step in VACUUM
. It is important to have
reasonably accurate statistics, otherwise poor choices of plans may
degrade database performance.
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 timestamp
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.
It is possible to run ANALYZE
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 PostgreSQL 7.2,
ANALYZE
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.
Tip
Although per-column tweaking of ANALYZE
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
ANALYZE
. Columns that are heavily used in WHERE
clauses
and have highly irregular data distributions may require a finer-grain
data histogram than other columns. See ALTER TABLE SET
STATISTICS
.
Recommended practice for most sites is to schedule a database-wide
ANALYZE
once a day at a low-usage time of day; this can
usefully be combined with a nightly VACUUM
. However,
sites with relatively slowly changing table statistics may find that
this is overkill, and that less-frequent ANALYZE
runs
are sufficient.
PostgreSQL's MVCC transaction semantics depend on being able to compare transaction ID (XID) numbers: a row version with an insertion XID greater than the current transaction's XID is “in the future” 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 transaction ID wraparound: the XID counter wraps around to zero, and all of a sudden transactions that were in the past appear to be in the future [mdash ] 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.)
Prior to PostgreSQL 7.2, the only defense
against XID wraparound was to re-initdb
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
initdb
or any sort of restart. The price is this
maintenance requirement: every table in the database must
be vacuumed at least once every billion transactions.
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 avoid disaster.
For each database in the cluster, PostgreSQL
keeps track of the time of the last database-wide VACUUM
.
When any database approaches the billion-transaction danger level,
the system begins to emit warning messages. If nothing is done, it
will eventually shut down normal operations until appropriate
manual maintenance is done. The remainder of this
section gives the details.
The new approach to XID comparison distinguishes two special XIDs,
numbers 1 and 2 (BootstrapXID
and
FrozenXID
). These two XIDs are always considered older
than every normal XID. Normal XIDs (those greater than 2) are
compared using modulo-231 arithmetic. This means
that for every normal XID, there are two billion XIDs that are
“older” and two billion that are “newer”; 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 “in the past” 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
FrozenXID
sometime before they reach the
two-billion-transactions-old mark. Once they are assigned this
special XID, they will appear to be “in the past” 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 VACUUM
.
VACUUM
's normal policy is to reassign FrozenXID
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 “frozen”.) With this policy,
the maximum safe interval between VACUUM
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 [mdash ] i.e., is lost to you. (Of course, it'll reappear
after another two billion transactions, but that's no help.)
Since periodic VACUUM
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, VACUUM
stores transaction ID
statistics in the system table pg_database
. In particular,
the datfrozenxid
column of a database's
pg_database
row is updated at the completion of any
database-wide VACUUM
operation (i.e.,
VACUUM
that does not
name a specific table). The value stored in this field is the freeze
cutoff XID that was used by that VACUUM
command. All normal
XIDs older than this cutoff XID are guaranteed to have been replaced by
FrozenXID
within that database. A convenient way to
examine this information is to execute the query
SELECT datname, age(datfrozenxid) FROM pg_database;
The age
column measures the number of transactions from the
cutoff XID to the current transaction's XID.
With the standard freezing policy, the age
column will start
at one billion for a freshly-vacuumed database. When the age
approaches two billion, the database must be vacuumed again to avoid
risk of wraparound failures. Recommended practice is to VACUUM
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 VACUUM
automatically delivers a warning
if there are any pg_database
entries showing an
age
of more than 1.5 billion transactions, for example:
play=# VACUUM; WARNING: database "mydb" must be vacuumed within 177009986 transactions HINT: To avoid a database shutdown, execute a full-database VACUUM in "mydb". VACUUM
If the warnings emitted by VACUUM
go ignored, then
PostgreSQL will begin to emit a warning
like the above on every transaction start once there are fewer than 10
million transactions left until wraparound. If those warnings also 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:
play=# select 2+2; ERROR: database is shut down to avoid wraparound data loss in database "mydb" HINT: Stop the postmaster and use a standalone backend to VACUUM in "mydb".
The 1-million-transaction safety margin exists to let the
administrator recover without data loss, by manually executing the
required VACUUM
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 postmaster and use a standalone
backend to execute VACUUM
. The shutdown mode is not enforced
by a standalone backend. See the postgres reference
page for details about using a standalone backend.
VACUUM
with the FREEZE
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
VACUUM FREEZE
is performed in an otherwise-idle
database, it is guaranteed that all 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
initdb
to prepare the template0
database.
It should also be used to prepare any user-created databases that
are to be marked datallowconn
= false
in
pg_database
, since there isn't any convenient way to
VACUUM
a database that you can't connect to.
Warning
A database that is marked datallowconn
= false
in pg_database
is assumed to be properly frozen; the
automatic warnings and wraparound protection shutdown do not take
such databases into account. Therefore it's up to you to ensure
you've correctly frozen a database before you mark it with
datallowconn
= false
.
Beginning in PostgreSQL 8.1, there is a
separate optional server process called the autovacuum
daemon, whose purpose is to automate the execution of
VACUUM
and ANALYZE
commands.
When enabled, the autovacuum daemon runs periodically and checks for
tables that have had a large number of inserted, updated or deleted
tuples. These checks use the row-level statistics collection facility;
therefore, the autovacuum daemon cannot be used unless stats_start_collector and stats_row_level are set to true
. Also,
it's important to allow a slot for the autovacuum process when choosing
the value of superuser_reserved_connections.
The autovacuum daemon, when enabled, runs every autovacuum_naptime seconds and determines which database
to process. Any database which is close to transaction ID wraparound
is immediately processed. In this case, autovacuum issues a
database-wide VACUUM
call, or VACUUM
FREEZE
if it's a template database, and then terminates. If
no database fulfills this criterion, the one that was least recently
processed by autovacuum is chosen. In this case each table in
the selected database is checked, and individual VACUUM
or ANALYZE
commands are issued as needed.
For each table, two conditions are used to determine which operation(s)
to apply. If the number of obsolete tuples since the last
VACUUM
exceeds the “vacuum threshold”, the
table is vacuumed. The vacuum threshold is defined as:
vacuum threshold = vacuum base threshold + vacuum scale factor * number of tuples
where the vacuum base threshold is
autovacuum_vacuum_threshold,
the vacuum scale factor is
autovacuum_vacuum_scale_factor,
and the number of tuples is
pg_class
.reltuples
.
The number of obsolete tuples is obtained from the statistics
collector; it is a semi-accurate count updated by each
UPDATE
and DELETE
operation. (It
is only semi-accurate because some information may be lost under heavy
load.) For analyze, a similar condition is used: the threshold, defined as
analyze threshold = analyze base threshold + analyze scale factor * number of tuples
is compared to the total number of tuples inserted, updated, or deleted
since the last ANALYZE
.
The default thresholds and scale factors are taken from
postgresql.conf
, but it is possible to override them
on a table-by-table basis by making entries in the system catalog
pg_autovacuum
.
If a pg_autovacuum
row exists for a particular
table, the settings it specifies are applied; otherwise the global
settings are used. See Section 17.9, “Automatic Vacuuming” for
more details on the global settings.
Besides the base threshold values and scale factors, there are three
more parameters that can be set for each table in
pg_autovacuum
.
The first, pg_autovacuum
.enabled
,
can be set to false
to instruct the autovacuum daemon
to skip that particular table entirely. In this case
autovacuum will only touch the table when it vacuums the entire database
to prevent transaction ID wraparound.
The other two parameters, the vacuum cost delay
(pg_autovacuum
.vac_cost_delay
)
and the vacuum cost limit
(pg_autovacuum
.vac_cost_limit
),
are used to set table-specific values for the
Cost-Based Vacuum Delay
feature.
If any of the values in pg_autovacuum
are set to a negative number, or if a row is not present at all in
pg_autovacuum
for any particular table, the
corresponding values from postgresql.conf
are used.
There is not currently any support for making
pg_autovacuum
entries, except by doing
manual INSERT
s into the catalog. This feature will be
improved in future releases, and it is likely that the catalog
definition will change.
Caution
The contents of the pg_autovacuum
system
catalog are currently not saved in database dumps created by
the tools pg_dump
and pg_dumpall
.
If you want to preserve them across a dump/reload cycle, make sure you
dump the catalog manually.