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| author | Peter Geoghegan <pg@bowt.ie> | 2022-04-02 12:29:35 -0700 |
|---|---|---|
| committer | Peter Geoghegan <pg@bowt.ie> | 2022-04-02 12:29:35 -0700 |
| commit | f7e4d5c64fb3977e3a773e7213472be1b59dab2f (patch) | |
| tree | 7f5c34101772af925ff8c8ebf9605f49fcccb61e | |
| parent | 1b208ebaf14e668a24c78ee42cf805431cd0f591 (diff) | |
| download | postgresql-f7e4d5c64fb3977e3a773e7213472be1b59dab2f.tar.gz postgresql-f7e4d5c64fb3977e3a773e7213472be1b59dab2f.zip | |
Doc: Clarify the role of aggressive VACUUMs.
Adjust the documentation's coverage of aggressive VACUUMs to make it
clearer that aggressive vacuuming isn't always strictly necessary. It's
possible for non-aggressive VACUUMs to advance relfrozenxid/relminmxid
without fail, given the right workload conditions.
While this has always been true, it matters more with recent and pending
improvements to VACUUM. These improvements make non-aggressive
vacuuming more likely to advance relfrozenxid/relminmxid in practice.
While this is an unrelated improvement to the docs, formally speaking,
it still doesn't seem worth backpatching. So don't backpatch.
| -rw-r--r-- | doc/src/sgml/maintenance.sgml | 14 |
1 files changed, 8 insertions, 6 deletions
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml index 693c92d9cba..5f3b6720e41 100644 --- a/doc/src/sgml/maintenance.sgml +++ b/doc/src/sgml/maintenance.sgml @@ -493,16 +493,17 @@ 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 <command>VACUUM</command>s won't always freeze every old row version in the table. - Periodically, <command>VACUUM</command> will perform an <firstterm>aggressive - vacuum</firstterm>, skipping only those pages which contain neither dead rows nor - any unfrozen XID or MXID values. + When that happens, <command>VACUUM</command> will eventually need to perform an + <firstterm>aggressive vacuum</firstterm>, 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. <xref linkend="guc-vacuum-freeze-table-age"/> controls when <command>VACUUM</command> 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 <varname>vacuum_freeze_table_age</varname> minus <varname>vacuum_freeze_min_age</varname>. Setting <varname>vacuum_freeze_table_age</varname> to 0 forces <command>VACUUM</command> to - use this more aggressive strategy for all scans. + always use its aggressive strategy. </para> <para> @@ -711,8 +712,9 @@ HINT: Stop the postmaster and vacuum that database in single-user mode. </para> <para> - Aggressive <command>VACUUM</command> scans, regardless of - what causes them, enable advancing the value for that table. + Aggressive <command>VACUUM</command>s, regardless of what causes + them, are <emphasis>guaranteed</emphasis> to be able to advance + the table's <structfield>relminmxid</structfield>. 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. |