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diff --git a/doc/src/sgml/html/routine-vacuuming.html b/doc/src/sgml/html/routine-vacuuming.html new file mode 100644 index 0000000..56f92b6 --- /dev/null +++ b/doc/src/sgml/html/routine-vacuuming.html @@ -0,0 +1,629 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>25.1. Routine Vacuuming</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="maintenance.html" title="Chapter 25. Routine Database Maintenance Tasks" /><link rel="next" href="routine-reindex.html" title="25.2. Routine Reindexing" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">25.1. Routine Vacuuming</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="maintenance.html" title="Chapter 25. Routine Database Maintenance Tasks">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="maintenance.html" title="Chapter 25. Routine Database Maintenance Tasks">Up</a></td><th width="60%" align="center">Chapter 25. Routine Database Maintenance Tasks</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="routine-reindex.html" title="25.2. Routine Reindexing">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="ROUTINE-VACUUMING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">25.1. Routine Vacuuming</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="routine-vacuuming.html#VACUUM-BASICS">25.1.1. Vacuuming Basics</a></span></dt><dt><span class="sect2"><a href="routine-vacuuming.html#VACUUM-FOR-SPACE-RECOVERY">25.1.2. Recovering Disk Space</a></span></dt><dt><span class="sect2"><a href="routine-vacuuming.html#VACUUM-FOR-STATISTICS">25.1.3. Updating Planner Statistics</a></span></dt><dt><span class="sect2"><a href="routine-vacuuming.html#VACUUM-FOR-VISIBILITY-MAP">25.1.4. Updating the Visibility Map</a></span></dt><dt><span class="sect2"><a href="routine-vacuuming.html#VACUUM-FOR-WRAPAROUND">25.1.5. Preventing Transaction ID Wraparound Failures</a></span></dt><dt><span class="sect2"><a href="routine-vacuuming.html#AUTOVACUUM">25.1.6. The Autovacuum Daemon</a></span></dt></dl></div><a id="id-1.6.12.10.2" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> databases require periodic + maintenance known as <em class="firstterm">vacuuming</em>. For many installations, it + is sufficient to let vacuuming be performed by the <em class="firstterm">autovacuum + daemon</em>, which is described in <a class="xref" href="routine-vacuuming.html#AUTOVACUUM" title="25.1.6. The Autovacuum Daemon">Section 25.1.6</a>. You might + need to adjust the autovacuuming parameters described there to obtain best + results for your situation. Some database administrators will want to + supplement or replace the daemon's activities with manually-managed + <code class="command">VACUUM</code> commands, which typically are executed according to a + schedule by <span class="application">cron</span> or <span class="application">Task + Scheduler</span> scripts. To set up manually-managed vacuuming properly, + it is essential to understand the issues discussed in the next few + subsections. Administrators who rely on autovacuuming may still wish + to skim this material to help them understand and adjust autovacuuming. + </p><div class="sect2" id="VACUUM-BASICS"><div class="titlepage"><div><div><h3 class="title">25.1.1. Vacuuming Basics</h3></div></div></div><p> + <span class="productname">PostgreSQL</span>'s + <a class="link" href="sql-vacuum.html" title="VACUUM"><code class="command">VACUUM</code></a> command has to + process each table on a regular basis for several reasons: + + </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">To recover or reuse disk space occupied by updated or deleted + rows.</li><li class="listitem">To update data statistics used by the + <span class="productname">PostgreSQL</span> query planner.</li><li class="listitem">To update the visibility map, which speeds + up <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes">index-only + scans</a>.</li><li class="listitem">To protect against loss of very old data due to + <em class="firstterm">transaction ID wraparound</em> or + <em class="firstterm">multixact ID wraparound</em>.</li></ol></div><p> + + Each of these reasons dictates performing <code class="command">VACUUM</code> operations + of varying frequency and scope, as explained in the following subsections. + </p><p> + There are two variants of <code class="command">VACUUM</code>: standard <code class="command">VACUUM</code> + and <code class="command">VACUUM FULL</code>. <code class="command">VACUUM FULL</code> can reclaim more + disk space but runs much more slowly. Also, + the standard form of <code class="command">VACUUM</code> can run in parallel with production + database operations. (Commands such as <code class="command">SELECT</code>, + <code class="command">INSERT</code>, <code class="command">UPDATE</code>, and + <code class="command">DELETE</code> will continue to function normally, though you + will not be able to modify the definition of a table with commands such as + <code class="command">ALTER TABLE</code> while it is being vacuumed.) + <code class="command">VACUUM FULL</code> requires an + <code class="literal">ACCESS EXCLUSIVE</code> 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 <code class="command">VACUUM</code> and + avoid <code class="command">VACUUM FULL</code>. + </p><p> + <code class="command">VACUUM</code> creates a substantial amount of I/O + traffic, which can cause poor performance for other active sessions. + There are configuration parameters that can be adjusted to reduce the + performance impact of background vacuuming — see + <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST" title="20.4.4. Cost-based Vacuum Delay">Section 20.4.4</a>. + </p></div><div class="sect2" id="VACUUM-FOR-SPACE-RECOVERY"><div class="titlepage"><div><div><h3 class="title">25.1.2. Recovering Disk Space</h3></div></div></div><a id="id-1.6.12.10.5.2" class="indexterm"></a><p> + In <span class="productname">PostgreSQL</span>, an + <code class="command">UPDATE</code> or <code class="command">DELETE</code> 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 (<acronym class="acronym">MVCC</acronym>, see <a class="xref" href="mvcc.html" title="Chapter 13. Concurrency Control">Chapter 13</a>): the row version + must not be deleted while it is still potentially visible to other + transactions. But eventually, an outdated or deleted row version is no + longer of interest to any transaction. The space it occupies must then be + reclaimed for reuse by new rows, to avoid unbounded growth of disk + space requirements. This is done by running <code class="command">VACUUM</code>. + </p><p> + The standard form of <code class="command">VACUUM</code> 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, <code class="command">VACUUM FULL</code> 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. + </p><p> + The usual goal of routine vacuuming is to do standard <code class="command">VACUUM</code>s + often enough to avoid needing <code class="command">VACUUM FULL</code>. The + autovacuum daemon attempts to work this way, and in fact will + never issue <code class="command">VACUUM FULL</code>. 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 <code class="command">VACUUM FULL</code> 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 <code class="command">VACUUM</code> runs are a + better approach than infrequent <code class="command">VACUUM FULL</code> runs for + maintaining heavily-updated tables. + </p><p> + Some administrators prefer to schedule vacuuming themselves, for example + doing all the work at night when load is low. + The difficulty with doing vacuuming according to a fixed schedule + is that if a table has an unexpected spike in update activity, it may + get bloated to the point that <code class="command">VACUUM FULL</code> 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 <code class="command">VACUUM</code>s are expected to do the bulk of the + work when the load is typical. + </p><p> + For those not using autovacuum, a typical approach is to schedule a + database-wide <code class="command">VACUUM</code> 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 + <code class="command">VACUUM</code> each one; the program <a class="xref" href="app-vacuumdb.html" title="vacuumdb"><span class="refentrytitle"><span class="application">vacuumdb</span></span></a> might be helpful. + </p><div class="tip"><h3 class="title">Tip</h3><p> + Plain <code class="command">VACUUM</code> 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 <code class="command">VACUUM FULL</code>, or alternatively + <a class="link" href="sql-cluster.html" title="CLUSTER"><code class="command">CLUSTER</code></a> + or one of the table-rewriting variants of + <a class="link" href="sql-altertable.html" title="ALTER TABLE"><code class="command">ALTER TABLE</code></a>. + These commands rewrite an entire new copy of the table and build + new indexes for it. All these options require an + <code class="literal">ACCESS EXCLUSIVE</code> 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. + </p></div><div class="tip"><h3 class="title">Tip</h3><p> + If you have a table whose entire contents are deleted on a periodic + basis, consider doing it with + <a class="link" href="sql-truncate.html" title="TRUNCATE"><code class="command">TRUNCATE</code></a> rather + than using <code class="command">DELETE</code> followed by + <code class="command">VACUUM</code>. <code class="command">TRUNCATE</code> removes the + entire content of the table immediately, without requiring a + subsequent <code class="command">VACUUM</code> or <code class="command">VACUUM + FULL</code> to reclaim the now-unused disk space. + The disadvantage is that strict MVCC semantics are violated. + </p></div></div><div class="sect2" id="VACUUM-FOR-STATISTICS"><div class="titlepage"><div><div><h3 class="title">25.1.3. Updating Planner Statistics</h3></div></div></div><a id="id-1.6.12.10.6.2" class="indexterm"></a><a id="id-1.6.12.10.6.3" class="indexterm"></a><p> + The <span class="productname">PostgreSQL</span> query planner relies on + statistical information about the contents of tables in order to + generate good plans for queries. These statistics are gathered by + the <a class="link" href="sql-analyze.html" title="ANALYZE"><code class="command">ANALYZE</code></a> command, + which can be invoked by itself or + as an optional step in <code class="command">VACUUM</code>. It is important to have + reasonably accurate statistics, otherwise poor choices of plans might + degrade database performance. + </p><p> + The autovacuum daemon, if enabled, will automatically issue + <code class="command">ANALYZE</code> commands whenever the content of a table has + changed sufficiently. However, administrators might prefer to rely + on manually-scheduled <code class="command">ANALYZE</code> operations, particularly + if it is known that update activity on a table will not affect the + statistics of <span class="quote">“<span class="quote">interesting</span>”</span> columns. The daemon schedules + <code class="command">ANALYZE</code> strictly as a function of the number of rows + inserted or updated; it has no knowledge of whether that will lead + to meaningful statistical changes. + </p><p> + 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 <code class="command">ANALYZE</code> on the parent table manually in order to + keep the statistics up to date. + </p><p> + As with vacuuming for space recovery, frequent updates of statistics + are more useful for heavily-updated tables than for seldom-updated + ones. But even for a heavily-updated table, there might be no need for + statistics updates if the statistical distribution of the data is + not changing much. A simple rule of thumb is to think about how much + the minimum and maximum values of the columns in the table change. + For example, a <code class="type">timestamp</code> column that contains the time + of row update will have a constantly-increasing maximum value as + rows are added and updated; such a column will probably need more + frequent statistics updates than, say, a column containing URLs for + pages accessed on a website. The URL column might receive changes just + as often, but the statistical distribution of its values probably + changes relatively slowly. + </p><p> + It is possible to run <code class="command">ANALYZE</code> 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. <code class="command">ANALYZE</code> uses a + statistically random sampling of the rows of a table rather than reading + every single row. + </p><div class="tip"><h3 class="title">Tip</h3><p> + Although per-column tweaking of <code class="command">ANALYZE</code> 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 + <code class="command">ANALYZE</code>. Columns that are heavily used in <code class="literal">WHERE</code> + clauses and have highly irregular data distributions might require a + finer-grain data histogram than other columns. See <code class="command">ALTER TABLE + SET STATISTICS</code>, or change the database-wide default using the <a class="xref" href="runtime-config-query.html#GUC-DEFAULT-STATISTICS-TARGET">default_statistics_target</a> configuration parameter. + </p><p> + 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. + </p></div><div class="tip"><h3 class="title">Tip</h3><p> + The autovacuum daemon does not issue <code class="command">ANALYZE</code> 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 + <code class="command">ANALYZE</code> commands on those tables on a suitable schedule. + </p></div><div class="tip"><h3 class="title">Tip</h3><p> + The autovacuum daemon does not issue <code class="command">ANALYZE</code> 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 <code class="command">ANALYZE</code> on those tables to keep the statistics + up to date. + </p></div></div><div class="sect2" id="VACUUM-FOR-VISIBILITY-MAP"><div class="titlepage"><div><div><h3 class="title">25.1.4. Updating the Visibility Map</h3></div></div></div><p> + Vacuum maintains a <a class="link" href="storage-vm.html" title="70.4. Visibility Map">visibility map</a> 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. + </p><p> + Second, it allows <span class="productname">PostgreSQL</span> to answer some + queries using only the index, without reference to the underlying table. + Since <span class="productname">PostgreSQL</span> 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 <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes"><em class="firstterm">index-only + scan</em></a>, 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. + </p></div><div class="sect2" id="VACUUM-FOR-WRAPAROUND"><div class="titlepage"><div><div><h3 class="title">25.1.5. Preventing Transaction ID Wraparound Failures</h3></div></div></div><a id="id-1.6.12.10.8.2" class="indexterm"></a><a id="id-1.6.12.10.8.3" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span>'s + <a class="link" href="mvcc-intro.html" title="13.1. Introduction">MVCC</a> 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">“<span class="quote">in the future</span>”</span> 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 <em class="firstterm">transaction ID + wraparound</em>: 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. + </p><p> + The reason that periodic vacuuming solves the problem is that + <code class="command">VACUUM</code> will mark rows as <span class="emphasis"><em>frozen</em></span>, 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-2<sup>32</sup> arithmetic. This means + that for every normal XID, there are two billion XIDs that are + <span class="quote">“<span class="quote">older</span>”</span> and two billion that are <span class="quote">“<span class="quote">newer</span>”</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">“<span class="quote">in the past</span>”</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 this, <span class="productname">PostgreSQL</span> reserves a special XID, + <code class="literal">FrozenTransactionId</code>, 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 + <code class="literal">FrozenTransactionId</code>, so that they will appear to be + <span class="quote">“<span class="quote">in the past</span>”</span> to all normal transactions regardless of wraparound + issues, and so such row versions will be valid until deleted, no matter + how long that is. + </p><div class="note"><h3 class="title">Note</h3><p> + In <span class="productname">PostgreSQL</span> versions before 9.4, freezing was + implemented by actually replacing a row's insertion XID + with <code class="literal">FrozenTransactionId</code>, which was visible in the + row's <code class="structname">xmin</code> system column. Newer versions just set a flag + bit, preserving the row's original <code class="structname">xmin</code> for possible + forensic use. However, rows with <code class="structname">xmin</code> equal + to <code class="literal">FrozenTransactionId</code> (2) may still be found + in databases <span class="application">pg_upgrade</span>'d from pre-9.4 versions. + </p><p> + Also, system catalogs may contain rows with <code class="structname">xmin</code> equal + to <code class="literal">BootstrapTransactionId</code> (1), indicating that they were + inserted during the first phase of <span class="application">initdb</span>. + Like <code class="literal">FrozenTransactionId</code>, this special XID is treated as + older than every normal XID. + </p></div><p> + <a class="xref" href="runtime-config-client.html#GUC-VACUUM-FREEZE-MIN-AGE">vacuum_freeze_min_age</a> + controls how old an XID value has to be before 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. + </p><p> + <code class="command">VACUUM</code> uses the <a class="link" href="storage-vm.html" title="70.4. Visibility Map">visibility map</a> + 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 + <code class="command">VACUUM</code>s won't always freeze every old row version in the table. + Periodically, <code class="command">VACUUM</code> will perform an <em class="firstterm">aggressive + vacuum</em>, skipping only those pages which contain neither dead rows nor + any unfrozen XID or MXID values. + <a class="xref" href="runtime-config-client.html#GUC-VACUUM-FREEZE-TABLE-AGE">vacuum_freeze_table_age</a> + controls when <code class="command">VACUUM</code> 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 <code class="varname">vacuum_freeze_table_age</code> minus + <code class="varname">vacuum_freeze_min_age</code>. Setting + <code class="varname">vacuum_freeze_table_age</code> to 0 forces <code class="command">VACUUM</code> to + use this more aggressive strategy for all scans. + </p><p> + The maximum time that a table can go unvacuumed is two billion + transactions minus the <code class="varname">vacuum_freeze_min_age</code> 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 <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-FREEZE-MAX-AGE">autovacuum_freeze_max_age</a>. (This will happen even if + autovacuum is disabled.) + </p><p> + This implies that if a table is not otherwise vacuumed, + autovacuum will be invoked on it approximately once every + <code class="varname">autovacuum_freeze_max_age</code> minus + <code class="varname">vacuum_freeze_min_age</code> 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 <code class="varname">autovacuum_freeze_max_age</code> or decreasing + <code class="varname">vacuum_freeze_min_age</code>. + </p><p> + The effective maximum for <code class="varname">vacuum_freeze_table_age</code> is 0.95 * + <code class="varname">autovacuum_freeze_max_age</code>; a setting higher than that will be + capped to the maximum. A value higher than + <code class="varname">autovacuum_freeze_max_age</code> 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 + <code class="command">VACUUM</code> before that happens. As a rule of thumb, + <code class="command">vacuum_freeze_table_age</code> should be set to a value somewhat + below <code class="varname">autovacuum_freeze_max_age</code>, leaving enough gap so that + a regularly scheduled <code class="command">VACUUM</code> 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. + </p><p> + The sole disadvantage of increasing <code class="varname">autovacuum_freeze_max_age</code> + (and <code class="varname">vacuum_freeze_table_age</code> along with it) is that + the <code class="filename">pg_xact</code> and <code class="filename">pg_commit_ts</code> + subdirectories of the database cluster will take more space, because it + must store the commit status and (if <code class="varname">track_commit_timestamp</code> is + enabled) timestamp of all transactions back to + the <code class="varname">autovacuum_freeze_max_age</code> horizon. The commit status uses + two bits per transaction, so if + <code class="varname">autovacuum_freeze_max_age</code> is set to its maximum allowed value + of two billion, <code class="filename">pg_xact</code> can be expected to grow to about half + a gigabyte and <code class="filename">pg_commit_ts</code> to about 20GB. If this + is trivial compared to your total database size, + setting <code class="varname">autovacuum_freeze_max_age</code> to its maximum allowed value + is recommended. Otherwise, set it depending on what you are willing to + allow for <code class="filename">pg_xact</code> and <code class="filename">pg_commit_ts</code> storage. + (The default, 200 million transactions, translates to about 50MB + of <code class="filename">pg_xact</code> storage and about 2GB of <code class="filename">pg_commit_ts</code> + storage.) + </p><p> + One disadvantage of decreasing <code class="varname">vacuum_freeze_min_age</code> is that + it might cause <code class="command">VACUUM</code> 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. + </p><p> + To track the age of the oldest unfrozen XIDs in a database, + <code class="command">VACUUM</code> stores XID + statistics in the system tables <code class="structname">pg_class</code> and + <code class="structname">pg_database</code>. In particular, + the <code class="structfield">relfrozenxid</code> column of a table's + <code class="structname">pg_class</code> row contains the freeze cutoff XID that was used + by the last aggressive <code class="command">VACUUM</code> for that table. All rows + inserted by transactions with XIDs older than this cutoff XID are + guaranteed to have been frozen. Similarly, + the <code class="structfield">datfrozenxid</code> column of a database's + <code class="structname">pg_database</code> row is a lower bound on the unfrozen XIDs + appearing in that database — it is just the minimum of the + per-table <code class="structfield">relfrozenxid</code> values within the database. + A convenient way to + examine this information is to execute queries such as: + +</p><pre class="programlisting"> +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; +</pre><p> + + The <code class="literal">age</code> column measures the number of transactions from the + cutoff XID to the current transaction's XID. + </p><p> + <code class="command">VACUUM</code> normally only scans pages that have been modified + since the last vacuum, but <code class="structfield">relfrozenxid</code> can only be + advanced when every page of the table + that might contain unfrozen XIDs is scanned. This happens when + <code class="structfield">relfrozenxid</code> is more than + <code class="varname">vacuum_freeze_table_age</code> transactions old, when + <code class="command">VACUUM</code>'s <code class="literal">FREEZE</code> option is used, or when all + pages that are not already all-frozen happen to + require vacuuming to remove dead row versions. When <code class="command">VACUUM</code> + scans every page in the table that is not already all-frozen, it should + set <code class="literal">age(relfrozenxid)</code> to a value just a little more than the + <code class="varname">vacuum_freeze_min_age</code> setting + that was used (more by the number of transactions started since the + <code class="command">VACUUM</code> started). If no <code class="structfield">relfrozenxid</code>-advancing + <code class="command">VACUUM</code> is issued on the table until + <code class="varname">autovacuum_freeze_max_age</code> is reached, an autovacuum will soon + be forced for the table. + </p><p> + If for some reason autovacuum fails to clear old XIDs from a table, the + system will begin to emit warning messages like this when the database's + oldest XIDs reach forty million transactions from the wraparound point: + +</p><pre class="programlisting"> +WARNING: database "mydb" must be vacuumed within 39985967 transactions +HINT: To avoid a database shutdown, execute a database-wide VACUUM in that database. +</pre><p> + + (A manual <code class="command">VACUUM</code> should fix the problem, as suggested by the + hint; but note that the <code class="command">VACUUM</code> must be performed by a + superuser, else it will fail to process system catalogs and thus not + be able to advance the database's <code class="structfield">datfrozenxid</code>.) + If these warnings are + ignored, the system will shut down and refuse to start any new + transactions once there are fewer than three million transactions left + until wraparound: + +</p><pre class="programlisting"> +ERROR: database is not accepting commands to avoid wraparound data loss in database "mydb" +HINT: Stop the postmaster and vacuum that database in single-user mode. +</pre><p> + + The three-million-transaction safety margin exists to let the + administrator recover without data loss, by manually executing the + required <code class="command">VACUUM</code> commands. However, since the system will not + execute commands once it has gone into the safety shutdown mode, + the only way to do this is to stop the server and start the server in single-user + mode to execute <code class="command">VACUUM</code>. The shutdown mode is not enforced + in single-user mode. See the <a class="xref" href="app-postgres.html" title="postgres"><span class="refentrytitle"><span class="application">postgres</span></span></a> reference + page for details about using single-user mode. + </p><div class="sect3" id="VACUUM-FOR-MULTIXACT-WRAPAROUND"><div class="titlepage"><div><div><h4 class="title">25.1.5.1. Multixacts and Wraparound</h4></div></div></div><a id="id-1.6.12.10.8.17.2" class="indexterm"></a><a id="id-1.6.12.10.8.17.3" class="indexterm"></a><p> + <em class="firstterm">Multixact IDs</em> 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 <span class="quote">“<span class="quote">multiple transaction ID</span>”</span>, 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 <code class="filename">pg_multixact</code> subdirectory, and only the multixact ID + appears in the <code class="structfield">xmax</code> 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. + </p><p> + Whenever <code class="command">VACUUM</code> scans any part of a table, it will replace + any multixact ID it encounters which is older than + <a class="xref" href="runtime-config-client.html#GUC-VACUUM-MULTIXACT-FREEZE-MIN-AGE">vacuum_multixact_freeze_min_age</a> + by a different value, which can be the zero value, a single + transaction ID, or a newer multixact ID. For each table, + <code class="structname">pg_class</code>.<code class="structfield">relminmxid</code> stores the oldest + possible multixact ID still appearing in any tuple of that table. + If this value is older than + <a class="xref" href="runtime-config-client.html#GUC-VACUUM-MULTIXACT-FREEZE-TABLE-AGE">vacuum_multixact_freeze_table_age</a>, 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. <code class="function">mxid_age()</code> can be used on + <code class="structname">pg_class</code>.<code class="structfield">relminmxid</code> to find its age. + </p><p> + Aggressive <code class="command">VACUUM</code> scans, regardless of + what causes them, enable advancing the value for that table. + 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. + </p><p> + As a safety device, an aggressive vacuum scan will + occur for any table whose multixact-age is greater than <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MULTIXACT-FREEZE-MAX-AGE">autovacuum_multixact_freeze_max_age</a>. 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. + </p></div></div><div class="sect2" id="AUTOVACUUM"><div class="titlepage"><div><div><h3 class="title">25.1.6. The Autovacuum Daemon</h3></div></div></div><a id="id-1.6.12.10.9.2" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> has an optional but highly + recommended feature called <em class="firstterm">autovacuum</em>, + whose purpose is to automate the execution of + <code class="command">VACUUM</code> and <code class="command">ANALYZE</code> commands. + When enabled, autovacuum checks for + tables that have had a large number of inserted, updated or deleted + tuples. These checks use the statistics collection facility; + therefore, autovacuum cannot be used unless <a class="xref" href="runtime-config-statistics.html#GUC-TRACK-COUNTS">track_counts</a> is set to <code class="literal">true</code>. + In the default configuration, autovacuuming is enabled and the related + configuration parameters are appropriately set. + </p><p> + The <span class="quote">“<span class="quote">autovacuum daemon</span>”</span> actually consists of multiple processes. + There is a persistent daemon process, called the + <em class="firstterm">autovacuum launcher</em>, which is in charge of starting + <em class="firstterm">autovacuum worker</em> processes for all databases. The + launcher will distribute the work across time, attempting to start one + worker within each database every <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-NAPTIME">autovacuum_naptime</a> + seconds. (Therefore, if the installation has <em class="replaceable"><code>N</code></em> databases, + a new worker will be launched every + <code class="varname">autovacuum_naptime</code>/<em class="replaceable"><code>N</code></em> seconds.) + A maximum of <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS">autovacuum_max_workers</a> worker processes + are allowed to run at the same time. If there are more than + <code class="varname">autovacuum_max_workers</code> 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 <code class="command">VACUUM</code> and/or <code class="command">ANALYZE</code> as needed. + <a class="xref" href="runtime-config-logging.html#GUC-LOG-AUTOVACUUM-MIN-DURATION">log_autovacuum_min_duration</a> can be set to monitor + autovacuum workers' activity. + </p><p> + 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 <a class="xref" href="runtime-config-connection.html#GUC-MAX-CONNECTIONS">max_connections</a> or + <a class="xref" href="runtime-config-connection.html#GUC-SUPERUSER-RESERVED-CONNECTIONS">superuser_reserved_connections</a> limits. + </p><p> + Tables whose <code class="structfield">relfrozenxid</code> value is more than + <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-FREEZE-MAX-AGE">autovacuum_freeze_max_age</a> transactions old are always + vacuumed (this also applies to those tables whose freeze max age has + been modified via storage parameters; see below). Otherwise, if the + number of tuples obsoleted since the last + <code class="command">VACUUM</code> exceeds the <span class="quote">“<span class="quote">vacuum threshold</span>”</span>, the + table is vacuumed. The vacuum threshold is defined as: +</p><pre class="programlisting"> +vacuum threshold = vacuum base threshold + vacuum scale factor * number of tuples +</pre><p> + where the vacuum base threshold is + <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-THRESHOLD">autovacuum_vacuum_threshold</a>, + the vacuum scale factor is + <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-SCALE-FACTOR">autovacuum_vacuum_scale_factor</a>, + and the number of tuples is + <code class="structname">pg_class</code>.<code class="structfield">reltuples</code>. + </p><p> + 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: +</p><pre class="programlisting"> +vacuum insert threshold = vacuum base insert threshold + vacuum insert scale factor * number of tuples +</pre><p> + where the vacuum insert base threshold is + <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-INSERT-THRESHOLD">autovacuum_vacuum_insert_threshold</a>, + and vacuum insert scale factor is + <a class="xref" href="runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-INSERT-SCALE-FACTOR">autovacuum_vacuum_insert_scale_factor</a>. + Such vacuums may allow portions of the table to be marked as + <em class="firstterm">all visible</em> and also allow tuples to be frozen, which + can reduce the work required in subsequent vacuums. + For tables which receive <code class="command">INSERT</code> operations but no or + almost no <code class="command">UPDATE</code>/<code class="command">DELETE</code> operations, + it may be beneficial to lower the table's + <a class="xref" href="sql-createtable.html#RELOPTION-AUTOVACUUM-FREEZE-MIN-AGE">autovacuum_freeze_min_age</a> 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 statistics collector; + it is a semi-accurate count updated by each <code class="command">UPDATE</code>, + <code class="command">DELETE</code> and <code class="command">INSERT</code> operation. (It is + only semi-accurate because some information might be lost under heavy + load.) If the <code class="structfield">relfrozenxid</code> value of the table + is more than <code class="varname">vacuum_freeze_table_age</code> transactions old, + an aggressive vacuum is performed to freeze old tuples and advance + <code class="structfield">relfrozenxid</code>; otherwise, only pages that have been modified + since the last vacuum are scanned. + </p><p> + For analyze, a similar condition is used: the threshold, defined as: +</p><pre class="programlisting"> +analyze threshold = analyze base threshold + analyze scale factor * number of tuples +</pre><p> + is compared to the total number of tuples inserted, updated, or deleted + since the last <code class="command">ANALYZE</code>. + </p><p> + Partitioned tables are not processed by autovacuum. Statistics + should be collected by running a manual <code class="command">ANALYZE</code> when it is + first populated, and again whenever the distribution of data in its + partitions changes significantly. + </p><p> + Temporary tables cannot be accessed by autovacuum. Therefore, + appropriate vacuum and analyze operations should be performed via + session SQL commands. + </p><p> + The default thresholds and scale factors are taken from + <code class="filename">postgresql.conf</code>, but it is possible to override them + (and many other autovacuum control parameters) on a per-table basis; see + <a class="xref" href="sql-createtable.html#SQL-CREATETABLE-STORAGE-PARAMETERS" title="Storage Parameters">Storage Parameters</a> 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 <a class="xref" href="runtime-config-autovacuum.html" title="20.10. Automatic Vacuuming">Section 20.10</a> for more details on + the global settings. + </p><p> + When multiple workers are running, the autovacuum cost delay parameters + (see <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST" title="20.4.4. Cost-based Vacuum Delay">Section 20.4.4</a>) are + <span class="quote">“<span class="quote">balanced</span>”</span> 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 <code class="literal">autovacuum_vacuum_cost_delay</code> or + <code class="literal">autovacuum_vacuum_cost_limit</code> storage parameters have been set + are not considered in the balancing algorithm. + </p><p> + Autovacuum workers generally don't block other commands. If a process + attempts to acquire a lock that conflicts with the + <code class="literal">SHARE UPDATE EXCLUSIVE</code> lock held by autovacuum, lock + acquisition will interrupt the autovacuum. For conflicting lock modes, + see <a class="xref" href="explicit-locking.html#TABLE-LOCK-COMPATIBILITY" title="Table 13.2. Conflicting Lock Modes">Table 13.2</a>. However, if the autovacuum + is running to prevent transaction ID wraparound (i.e., the autovacuum query + name in the <code class="structname">pg_stat_activity</code> view ends with + <code class="literal">(to prevent wraparound)</code>), the autovacuum is not + automatically interrupted. + </p><div class="warning"><h3 class="title">Warning</h3><p> + Regularly running commands that acquire locks conflicting with a + <code class="literal">SHARE UPDATE EXCLUSIVE</code> lock (e.g., ANALYZE) can + effectively prevent autovacuums from ever completing. + </p></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="maintenance.html" title="Chapter 25. Routine Database Maintenance Tasks">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="maintenance.html" title="Chapter 25. 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