PostgreSQL™ databases require periodic maintenance known as vacuuming. For many installations, it is sufficient to let vacuuming be performed by the autovacuum daemon, which is described in Section 24.1.6. 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 VACUUM commands, which typically are executed according to a schedule by cron or Task Scheduler 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.
PostgreSQL™'s VACUUM command has to process each table on a regular basis for several reasons:
Each of these reasons dictates performing VACUUM operations of varying frequency and scope, as explained in the following subsections.
There are two variants of VACUUM: standard VACUUM
and VACUUM FULL. VACUUM FULL can reclaim more
disk space but runs much more slowly. Also,
the standard form of VACUUM can run in parallel with production
database operations. (Commands such as SELECT,
INSERT, UPDATE, and
DELETE will continue to function normally, though you
will not be able to modify the definition of a table with commands such as
ALTER TABLE while it is being vacuumed.)
VACUUM FULL requires an
ACCESS EXCLUSIVE
lock on the table it is
working on, and therefore cannot be done in parallel with other use
of the table. Generally, therefore,
administrators should strive to use standard VACUUM and
avoid VACUUM FULL.
VACUUM 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 Section 19.4.4.
In PostgreSQL™, 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 (MVCC, see Chapter 13): 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 But eventually, an outdated or deleted row version is no longer of interest to any transaction.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 unbounded growth of disk space requirements. This is done by running VACUUM.
The standard form of VACUUM 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, VACUUM FULL actively compacts tables by writing a complete new version of the table file with no dead space. This minimizes the size of the table, but can take a long time. It also requires extra disk space for the new copy of the table, until the operation completes.
The usual goal of routine vacuuming is to do standard VACUUMs often enough to avoid needing VACUUM FULL. The autovacuum daemon attempts to work this way, and in fact will never issue VACUUM FULL. 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 vacuum runs. Although VACUUM FULL 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 VACUUM runs are a better approach than infrequent VACUUM FULL runs for maintaining heavily-updated tables.
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 VACUUM FULL 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 VACUUMs are expected to do the bulk of the work when the load is typical.
For those not using autovacuum, a typical approach is to schedule a database-wide VACUUM 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 VACUUM each one; the program vacuumdb might be helpful.
Plain VACUUM may not be 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, you will need
to use VACUUM FULL, or alternatively
CLUSTER
or one of the table-rewriting variants of
ALTER TABLE.
These commands rewrite an entire new copy of the table and build
new indexes for it. All these options require an
ACCESS EXCLUSIVE
lock. Note that
they also temporarily use extra disk space approximately equal to the size
of the table, since the old copies of the table and indexes can't be
released until the new ones are complete.
If you have a table whose entire 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 disadvantage is that strict MVCC semantics are violated.
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 might degrade database performance.
The autovacuum daemon, if enabled, will automatically issue ANALYZE commands whenever the content of a table has changed sufficiently. However, administrators might prefer to rely on manually-scheduled ANALYZE operations, particularly if it is known that update activity on a table will not affect the statistics of “interesting” columns. The daemon schedules ANALYZE 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.
Tuples changed in partitions and inheritance children do not trigger analyze on the parent table. If the parent table is empty or rarely changed, it may never be processed by autovacuum, and the statistics for the inheritance tree as a whole won't be collected. It is necessary to run ANALYZE on the parent table manually in order to keep the statistics up to date.
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 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 might 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, it is usually best to just analyze the entire database, because it is a fast operation. ANALYZE uses a statistically random sampling of the rows of a table rather than reading every single row.
Although per-column tweaking of ANALYZE frequency might not be
very productive, you might 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 might require a
finer-grain data histogram than other columns. See ALTER TABLE
SET STATISTICS, or change the database-wide default using the default_statistics_target configuration parameter.
Also, by default there is limited information available about the selectivity of functions. However, if you create a statistics object or an expression index that uses a function call, useful statistics will be gathered about the function, which can greatly improve query plans that use the expression index.
The autovacuum daemon does not issue ANALYZE commands for foreign tables, since it has no means of determining how often that might be useful. If your queries require statistics on foreign tables for proper planning, it's a good idea to run manually-managed ANALYZE commands on those tables on a suitable schedule.
The autovacuum daemon does not issue ANALYZE commands for partitioned tables. Inheritance parents will only be analyzed if the parent itself is changed - changes to child tables do not trigger autoanalyze on the parent table. If your queries require statistics on parent tables for proper planning, it is necessary to periodically run a manual ANALYZE on those tables to keep the statistics up to date.
Vacuum maintains a visibility map for each table to keep track of which pages contain only tuples that are known to be visible to all active transactions (and all future transactions, until the page is again modified). This has two purposes. First, vacuum itself can skip such pages on the next run, since there is nothing to clean up.
Second, it allows PostgreSQL™ to answer some queries using only the index, without reference to the underlying table. Since PostgreSQL™ indexes don't contain tuple visibility information, a normal index scan fetches the heap tuple for each matching index entry, to check whether it should be seen by the current transaction. An index-only scan, on the other hand, checks the visibility map first. If it's known that all tuples on the page are visible, the heap fetch can be skipped. This is most useful on large data sets where the visibility map can prevent disk accesses. The visibility map is vastly smaller than the heap, so it can easily be cached even when the heap is very large.
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) 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 — which means their output 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.
The reason that periodic vacuuming solves the problem is that
VACUUM will mark rows as frozen, indicating that
they were inserted by a transaction that committed sufficiently far in
the past that the effects of the inserting transaction are certain to be
visible to all current and future transactions.
Normal XIDs are
compared using modulo-232 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 this, PostgreSQL™ reserves a special XID,
FrozenTransactionId
, which does not follow the normal XID
comparison rules and is always considered older
than every normal XID.
Frozen row versions are treated as if the inserting XID were
FrozenTransactionId
, so that they will appear to be
“in the past” to all normal transactions regardless of wraparound
issues, and so such row versions will be valid until deleted, no matter
how long that is.
In PostgreSQL™ versions before 9.4, freezing was
implemented by actually replacing a row's insertion XID
with FrozenTransactionId
, which was visible in the
row's xmin system column. Newer versions just set a flag
bit, preserving the row's original xmin for possible
forensic use. However, rows with xmin equal
to FrozenTransactionId
(2) may still be found
in databases pg_upgrade'd from pre-9.4 versions.
Also, system catalogs may contain rows with xmin equal
to BootstrapTransactionId
(1), indicating that they were
inserted during the first phase of initdb.
Like FrozenTransactionId
, this special XID is treated as
older than every normal XID.
vacuum_freeze_min_age controls how old an XID value has to be before rows bearing that XID will be frozen. Increasing this setting may avoid unnecessary work if the rows that would otherwise be frozen will soon be modified again, but decreasing this setting increases the number of transactions that can elapse before the table must be vacuumed again.
VACUUM uses the visibility map
to determine which pages of a table must be scanned. Normally, it
will skip pages that don't have any dead row versions even if those pages
might still have row versions with old XID values. Therefore, normal
VACUUMs won't always freeze every old row version in the table.
When that happens, VACUUM will eventually need to perform an
aggressive vacuum, which will freeze all eligible unfrozen
XID and MXID values, including those from all-visible but not all-frozen pages.
In practice most tables require periodic aggressive vacuuming.
vacuum_freeze_table_age
controls when VACUUM does that: all-visible but not all-frozen
pages are scanned if the number of transactions that have passed since the
last such scan is greater than vacuum_freeze_table_age
minus
vacuum_freeze_min_age
. Setting
vacuum_freeze_table_age
to 0 forces VACUUM to
always use its aggressive strategy.
The maximum time that a table can go unvacuumed is two billion
transactions minus the vacuum_freeze_min_age
value at
the time of the last aggressive vacuum. 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 unfrozen rows with
XIDs older than the age specified by the configuration parameter autovacuum_freeze_max_age. (This will happen even if
autovacuum is disabled.)
This implies that if a table is not otherwise vacuumed,
autovacuum will be invoked on it approximately once every
autovacuum_freeze_max_age
minus
vacuum_freeze_min_age
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 to vacuum for space reclamation, 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 autovacuum_freeze_max_age
or decreasing
vacuum_freeze_min_age
.
The effective maximum for vacuum_freeze_table_age
is 0.95 *
autovacuum_freeze_max_age
; a setting higher than that will be
capped to the maximum. A value higher than
autovacuum_freeze_max_age
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
VACUUM before that happens. As a rule of thumb,
vacuum_freeze_table_age should be set to a value somewhat
below autovacuum_freeze_max_age
, leaving enough gap so that
a regularly scheduled VACUUM 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 aggressive vacuuming.
The sole disadvantage of increasing autovacuum_freeze_max_age
(and vacuum_freeze_table_age
along with it) is that
the pg_xact
and pg_commit_ts
subdirectories of the database cluster will take more space, because it
must store the commit status and (if track_commit_timestamp
is
enabled) timestamp of all transactions back to
the autovacuum_freeze_max_age
horizon. The commit status uses
two bits per transaction, so if
autovacuum_freeze_max_age
is set to its maximum allowed value
of two billion, pg_xact
can be expected to grow to about half
a gigabyte and pg_commit_ts
to about 20GB. If this
is trivial compared to your total database size,
setting autovacuum_freeze_max_age
to its maximum allowed value
is recommended. Otherwise, set it depending on what you are willing to
allow for pg_xact
and pg_commit_ts
storage.
(The default, 200 million transactions, translates to about 50MB
of pg_xact
storage and about 2GB of pg_commit_ts
storage.)
One disadvantage of decreasing vacuum_freeze_min_age
is that
it might cause VACUUM to do useless work: freezing a row
version 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.
To track the age of the oldest unfrozen XIDs in a database,
VACUUM stores XID
statistics in the system tables pg_class and
pg_database. In particular,
the relfrozenxid
column of a table's
pg_class row contains the oldest remaining unfrozen
XID at the end of the most recent VACUUM that successfully
advanced relfrozenxid
(typically the most recent
aggressive VACUUM). Similarly, the
datfrozenxid
column of a database's
pg_database row is a lower bound on the unfrozen XIDs
appearing in that database — it is just the minimum of the
per-table relfrozenxid
values within the database.
A convenient way to
examine this information is to execute queries such as:
SELECT c.oid::regclass as table_name, greatest(age(c.relfrozenxid),age(t.relfrozenxid)) as age FROM pg_class c LEFT JOIN pg_class t ON c.reltoastrelid = t.oid WHERE c.relkind IN ('r', 'm'); 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.
When the VACUUM command's VERBOSE
parameter is specified, VACUUM prints various
statistics about the table. This includes information about how
relfrozenxid
and
relminmxid
advanced, and the number of
newly frozen pages. The same details appear in the server log when
autovacuum logging (controlled by log_autovacuum_min_duration) reports on a
VACUUM operation executed by autovacuum.
VACUUM normally only scans pages that have been modified
since the last vacuum, but relfrozenxid
can only be
advanced when every page of the table
that might contain unfrozen XIDs is scanned. This happens when
relfrozenxid
is more than
vacuum_freeze_table_age
transactions old, when
VACUUM's FREEZE
option is used, or when all
pages that are not already all-frozen happen to
require vacuuming to remove dead row versions. When VACUUM
scans every page in the table that is not already all-frozen, it should
set age(relfrozenxid)
to a value just a little more than the
vacuum_freeze_min_age
setting
that was used (more by the number of transactions started since the
VACUUM started). VACUUM
will set relfrozenxid
to the oldest XID
that remains in the table, so it's possible that the final value
will be much more recent than strictly required.
If no relfrozenxid
-advancing
VACUUM is issued on the table until
autovacuum_freeze_max_age
is reached, an autovacuum will soon
be forced for the table.
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 forty million transactions from the wraparound point:
WARNING: database "mydb" must be vacuumed within 39985967 transactions HINT: To avoid XID assignment failures, execute a database-wide VACUUM in that database.
(A manual VACUUM should fix the problem, as suggested by the
hint; but note that the VACUUM should be performed by a
superuser, else it will fail to process system catalogs, which prevent it from
being able to advance the database's datfrozenxid
.)
If these warnings are ignored, the system will refuse to assign new XIDs once
there are fewer than three million transactions left until wraparound:
ERROR: database is not accepting commands that assign new XIDs to avoid wraparound data loss in database "mydb" HINT: Execute a database-wide VACUUM in that database.
In this condition any transactions already in progress can continue, but only read-only transactions can be started. Operations that modify database records or truncate relations will fail. The VACUUM command can still be run normally. Note that, contrary to what was sometimes recommended in earlier releases, it is not necessary or desirable to stop the postmaster or enter single user-mode in order to restore normal operation. Instead, follow these steps:
age(transactionid)
is large. Such transactions should be
committed or rolled back.
age(backend_xid)
or age(backend_xmin)
is
large. Such transactions should be committed or rolled back, or the session
can be terminated using pg_terminate_backend
.
age(xmin)
or age(catalog_xmin)
is large. In many cases, such slots were created for replication to servers that no
longer exist, or that have been down for a long time. If you drop a slot for a server
that still exists and might still try to connect to that slot, that replica may
need to be rebuilt.
VACUUM
is simplest; to reduce the time required, it as also possible
to issue manual VACUUM commands on the tables where
relminxid
is oldest. Do not use VACUUM FULL
in this scenario, because it requires an XID and will therefore fail, except in super-user
mode, where it will instead consume an XID and thus increase the risk of transaction ID
wraparound. Do not use VACUUM FREEZE
either, because it will do
more than the minimum amount of work required to restore normal operation.
In earlier versions, it was sometimes necessary to stop the postmaster and VACUUM the database in a single-user mode. In typical scenarios, this is no longer necessary, and should be avoided whenever possible, since it involves taking the system down. It is also riskier, since it disables transaction ID wraparound safeguards that are designed to prevent data loss. The only reason to use single-user mode in this scenario is if you wish to TRUNCATE or DROP unneeded tables to avoid needing to VACUUM them. The three-million-transaction safety margin exists to let the administrator do this. See the postgres reference page for details about using single-user mode.
Multixact IDs are used to support row locking by
multiple transactions. Since there is only limited space in a tuple
header to store lock information, that information is encoded as
a “multiple transaction ID”, or multixact ID for short,
whenever there is more than one transaction concurrently locking a
row. Information about which transaction IDs are included in any
particular multixact ID is stored separately in
the pg_multixact
subdirectory, and only the multixact ID
appears in the xmax
field in the tuple header.
Like transaction IDs, multixact IDs are implemented as a
32-bit counter and corresponding storage, all of which requires
careful aging management, storage cleanup, and wraparound handling.
There is a separate storage area which holds the list of members in
each multixact, which also uses a 32-bit counter and which must also
be managed.
Whenever VACUUM scans any part of a table, it will replace
any multixact ID it encounters which is older than
vacuum_multixact_freeze_min_age
by a different value, which can be the zero value, a single
transaction ID, or a newer multixact ID. For each table,
pg_class.relminmxid
stores the oldest
possible multixact ID still appearing in any tuple of that table.
If this value is older than
vacuum_multixact_freeze_table_age, an aggressive
vacuum is forced. As discussed in the previous section, an aggressive
vacuum means that only those pages which are known to be all-frozen will
be skipped. mxid_age()
can be used on
pg_class.relminmxid
to find its age.
Aggressive VACUUMs, regardless of what causes
them, are guaranteed to be able to advance
the table's relminmxid
.
Eventually, as all tables in all databases are scanned and their
oldest multixact values are advanced, on-disk storage for older
multixacts can be removed.
As a safety device, an aggressive vacuum scan will occur for any table whose multixact-age is greater than autovacuum_multixact_freeze_max_age. Also, if the storage occupied by multixacts members exceeds about 10GB, aggressive vacuum scans will occur more often for all tables, starting with those that have the oldest multixact-age. Both of these kinds of aggressive scans will occur even if autovacuum is nominally disabled. The members storage area can grow up to about 20GB before reaching wraparound.
Similar to the XID case, if autovacuum fails to clear old MXIDs from a table, the system will begin to emit warning messages when the database's oldest MXIDs reach forty million transactions from the wraparound point. And, just as in the XID case, if these warnings are ignored, the system will refuse to generate new MXIDs once there are fewer than three million left until wraparound.
Normal operation when MXIDs are exhausted can be restored in much the same way as when XIDs are exhausted. Follow the same steps in the previous section, but with the following differences:
pg_stat_activity
; however, looking for old XIDs is still a good
way of determining which transactions are causing MXID wraparound problems.
PostgreSQL™ has an optional but highly
recommended feature called autovacuum,
whose purpose is to automate the execution of
VACUUM and ANALYZE 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 track_counts is set to true
.
In the default configuration, autovacuuming is enabled and the related
configuration parameters are appropriately set.
The “autovacuum daemon” actually consists of multiple processes.
There is a persistent daemon process, called the
autovacuum launcher, which is in charge of starting
autovacuum worker processes for all databases. The
launcher will distribute the work across time, attempting to start one
worker within each database every autovacuum_naptime
seconds. (Therefore, if the installation has N
databases,
a new worker will be launched every
autovacuum_naptime
/N
seconds.)
A maximum of autovacuum_max_workers worker processes
are allowed to run at the same time. If there are more than
autovacuum_max_workers
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 VACUUM and/or ANALYZE as needed.
log_autovacuum_min_duration can be set to monitor
autovacuum workers' activity.
If several large tables all become eligible for vacuuming in a short amount of time, all autovacuum workers might become occupied with vacuuming those tables for a long period. This would result in other tables and databases not being vacuumed until a worker becomes available. There is no 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 max_connections or superuser_reserved_connections limits.
Tables whose relfrozenxid
value is more than
autovacuum_freeze_max_age 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
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 table is also vacuumed if the number of tuples inserted since the last vacuum has exceeded the defined insert threshold, which is defined as:
vacuum insert threshold = vacuum base insert threshold + vacuum insert scale factor * number of tuples
where the vacuum insert base threshold is
autovacuum_vacuum_insert_threshold,
and vacuum insert scale factor is
autovacuum_vacuum_insert_scale_factor.
Such vacuums may allow portions of the table to be marked as
all visible and also allow tuples to be frozen, which
can reduce the work required in subsequent vacuums.
For tables which receive INSERT operations but no or
almost no UPDATE/DELETE operations,
it may be beneficial to lower the table's
autovacuum_freeze_min_age as this may allow
tuples to be frozen by earlier vacuums. The number of obsolete tuples and
the number of inserted tuples are obtained from the cumulative statistics system;
it is an eventually-consistent count updated by each UPDATE,
DELETE and INSERT operation.
If the relfrozenxid
value of the table
is more than vacuum_freeze_table_age
transactions old,
an aggressive vacuum is performed to freeze old tuples and advance
relfrozenxid
; otherwise, only pages that have been modified
since the last vacuum are scanned.
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.
Partitioned tables do not directly store tuples and consequently are not processed by autovacuum. (Autovacuum does process table partitions just like other tables.) Unfortunately, this means that autovacuum does not run ANALYZE on partitioned tables, and this can cause suboptimal plans for queries that reference partitioned table statistics. You can work around this problem by manually running ANALYZE on partitioned tables when they are first populated, and again whenever the distribution of data in their partitions changes significantly.
Temporary tables cannot be accessed by autovacuum. Therefore, appropriate vacuum and analyze operations should be performed via session SQL commands.
The default thresholds and scale factors are taken from
postgresql.conf
, but it is possible to override them
(and many other autovacuum control parameters) on a per-table basis; see
Storage Parameters for more information.
If a setting has been changed via a table's storage parameters, that value
is used when processing that table; otherwise the global settings are
used. See Section 19.10 for more details on
the global settings.
When multiple workers are running, the autovacuum cost delay parameters
(see Section 19.4.4) are
“balanced” among all the running workers, so that the
total I/O impact on the system is the same regardless of the number
of workers actually running. However, any workers processing tables whose
per-table autovacuum_vacuum_cost_delay
or
autovacuum_vacuum_cost_limit
storage parameters have been set
are not considered in the balancing algorithm.
Autovacuum workers generally don't block other commands. If a process
attempts to acquire a lock that conflicts with the
SHARE UPDATE EXCLUSIVE
lock held by autovacuum, lock
acquisition will interrupt the autovacuum. For conflicting lock modes,
see Table 13.2. However, if the autovacuum
is running to prevent transaction ID wraparound (i.e., the autovacuum query
name in the pg_stat_activity view ends with
(to prevent wraparound)
), the autovacuum is not
automatically interrupted.
Regularly running commands that acquire locks conflicting with a
SHARE UPDATE EXCLUSIVE
lock (e.g., ANALYZE) can
effectively prevent autovacuums from ever completing.