25.1. Routine Vacuuming

25.1. Routine Vacuuming
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25.1.1. Vacuuming Basics
25.1.2. Recovering Disk Space
25.1.3. Updating Planner Statistics
25.1.4. Updating the Visibility Map
25.1.5. Preventing Transaction ID Wraparound Failures
25.1.6. The Autovacuum Daemon

+ 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 25.1.6. 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. +

25.1.1. Vacuuming Basics

+ PostgreSQL's + VACUUM command has to + process each table on a regular basis for several reasons: + +

To recover or reuse disk space occupied by updated or deleted + rows.
To update data statistics used by the + PostgreSQL query planner.
To update the visibility map, which speeds + up index-only + scans.
To protect against loss of very old data due to + transaction ID wraparound or + multixact ID wraparound.

+ + 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 20.4.4. +

25.1.2. Recovering Disk Space

+ 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. +

Tip

+ 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. +

Tip

+ 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. +

25.1.3. Updating Planner Statistics

+ 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. +

Tip

+ 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. +

Tip

+ 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. +

Tip

+ 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. +

25.1.4. Updating the Visibility Map

+ 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. +

25.1.5. Preventing Transaction ID Wraparound Failures

+ 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. +

Note

+ 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. +

Tip

+ 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. 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 a database shutdown, 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 to avoid wraparound data loss in database "mydb"
+HINT:  Stop the postmaster and vacuum that database in single-user mode.
+

+ + 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. + Contrary to what the hint states, 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: + +

Resolve old prepared transactions. You can find these by checking + pg_prepared_xacts for rows where + age(transactionid) is large. Such transactions should be + committed or rolled back.
End long-running open transactions. You can find these by checking + pg_stat_activity for rows where + 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.
Drop any old replication slots. Use + pg_stat_replication to + find slots where 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.
Execute VACUUM in the target database. A database-wide + 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.
Once normal operation is restored, ensure that autovacuum is properly configured + in the target database in order to avoid future problems.

Note

+ 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. +

25.1.5.1. Multixacts and Wraparound

+ 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 2GB, 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. +

+ 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 an 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: + +

Running transactions and prepared transactions can be ignored if there + is no chance that they might appear in a multixact.
MXID information is not directly visible in system views such as + pg_stat_activity; however, looking for old XIDs is still a good + way of determining which transactions are causing MXID wraparound problems.
XID exhaustion will block all write transactions, but MXID exhaustion will + only block a subset of write transactions, specifically those that involve + row locks that require an MXID.

25.1.6. The Autovacuum Daemon

+ 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 a semi-accurate count updated by each UPDATE, + DELETE and INSERT operation. (It is + only semi-accurate because some information might be lost under heavy + load.) 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 20.10 for more details on + the global settings. +

+ When multiple workers are running, the autovacuum cost delay parameters + (see Section 20.4.4) 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. +

Warning

+ Regularly running commands that acquire locks conflicting with a + SHARE UPDATE EXCLUSIVE lock (e.g., ANALYZE) can + effectively prevent autovacuums from ever completing. +