diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/pgvisibility.sgml | 30 |
1 files changed, 24 insertions, 6 deletions
diff --git a/doc/src/sgml/pgvisibility.sgml b/doc/src/sgml/pgvisibility.sgml index 4cdca7dada9..44e83de7289 100644 --- a/doc/src/sgml/pgvisibility.sgml +++ b/doc/src/sgml/pgvisibility.sgml @@ -9,14 +9,16 @@ <para> The <filename>pg_visibility</> module provides a means for examining the - visibility map (VM) and page-level visibility information. + visibility map (VM) and page-level visibility information. It also + provides functions to check the integrity of the visibility map and to + force it to be rebuilt. </para> <para> - These routines return information about three different bits. The - all-visible bit in the visibility map indicates that every tuple on - a given page of a relation is visible to every current transaction. The - all-frozen bit in the visibility map indicates that every tuple on the + Three different bits are used to store information about page-level + visibility. The all-visible bit in the visibility map indicates that every + tuple on a given page of a relation is visible to every current transaction. + The all-frozen bit in the visibility map indicates that every tuple on the page is frozen; that is, no future vacuum will need to modify the page until such time as a tuple is inserted, updated, deleted, or locked on that page. The page-level <literal>PD_ALL_VISIBLE</literal> bit has the @@ -25,7 +27,8 @@ will normally agree, but the page-level bit can sometimes be set while the visibility map bit is clear after a crash recovery; or they can disagree because of a change which occurs after <literal>pg_visibility</> examines - the visibility map and before it examines the data page. + the visibility map and before it examines the data page. Any event which + causes data corruption can also cause these bits to disagree. </para> <para> @@ -118,6 +121,21 @@ </para> </listitem> </varlistentry> + + <varlistentry> + <term><function>pg_truncate_visibility_map(regclass) returns void</function></term> + + <listitem> + <para> + Truncates the visibility map for the given relation. This function + is only expected to be useful if you suspect that the visibility map + for the indicated relation is corrupt and wish to rebuild it. The first + <command>VACUUM</> executed on the given relation after this function + is executed will scan every page in the relation and rebuild the + visibility map. + </para> + </listitem> + </varlistentry> </variablelist> <para> |