<para>
The <filename>pg_visibility</> module provides a means for examining the
- visibility map (VM) and page-level visibility information. It also
- provides functions to check the integrity of the visibility map and to
+ visibility map (VM) and page-level visibility information of a table.
+ It also provides functions to check the integrity of a visibility map and to
force it to be rebuilt.
</para>
<para>
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
+ tuple in the corresponding page of the relation is visible to every current
+ and future transaction. The all-frozen bit in the visibility map indicates
+ that every tuple in 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 header's <literal>PD_ALL_VISIBLE</literal> bit has the
same meaning as the all-visible bit in the visibility map, but is stored
- within the data page itself rather than a separate data structure. These
- 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. Any event which
- causes data corruption can also cause these bits to disagree.
+ within the data page itself rather than in a separate data structure.
+ These two bits will normally agree, but the page's all-visible bit can
+ sometimes be set while the visibility map bit is clear after a crash
+ recovery. The reported values can also disagree because of a change that
+ occurs after <literal>pg_visibility</> examines the visibility map and
+ before it examines the data page. Any event that causes data corruption
+ can also cause these bits to disagree.
</para>
<para>
- Functions which display information about <literal>PD_ALL_VISIBLE</>
- are much more costly than those which only consult the visibility map,
+ Functions that display information about <literal>PD_ALL_VISIBLE</> bits
+ are much more costly than those that only consult the visibility map,
because they must read the relation's data blocks rather than only the
(much smaller) visibility map. Functions that check the relation's
data blocks are similarly expensive.
<variablelist>
<varlistentry>
- <term><function>pg_visibility_map(regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean) returns record</function></term>
+ <term><function>pg_visibility_map(relation regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean) returns record</function></term>
<listitem>
<para>
Returns the all-visible and all-frozen bits in the visibility map for
</varlistentry>
<varlistentry>
- <term><function>pg_visibility(regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns record</function></term>
+ <term><function>pg_visibility(relation regclass, blkno bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns record</function></term>
<listitem>
<para>
Returns the all-visible and all-frozen bits in the visibility map for
the given block of the given relation, plus the
- <literal>PD_ALL_VISIBLE</> bit for that block.
+ <literal>PD_ALL_VISIBLE</> bit of that block.
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><function>pg_visibility_map(regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean) returns setof record</function></term>
+ <term><function>pg_visibility_map(relation regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean) returns setof record</function></term>
<listitem>
<para>
Returns the all-visible and all-frozen bits in the visibility map for
- each block the given relation.
+ each block of the given relation.
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><function>pg_visibility(regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns setof record</function></term>
+ <term><function>pg_visibility(relation regclass, blkno OUT bigint, all_visible OUT boolean, all_frozen OUT boolean, pd_all_visible OUT boolean) returns setof record</function></term>
<listitem>
<para>
Returns the all-visible and all-frozen bits in the visibility map for
- each block the given relation, plus the <literal>PD_ALL_VISIBLE</>
- bit for each block.
+ each block of the given relation, plus the <literal>PD_ALL_VISIBLE</>
+ bit of each block.
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><function>pg_visibility_map_summary(regclass, all_visible OUT bigint, all_frozen OUT bigint) returns record</function></term>
+ <term><function>pg_visibility_map_summary(relation regclass, all_visible OUT bigint, all_frozen OUT bigint) returns record</function></term>
<listitem>
<para>
</para>
</listitem>
</varlistentry>
-
+
<varlistentry>
- <term><function>pg_check_frozen(regclass, t_ctid OUT tid) returns setof tid</function></term>
+ <term><function>pg_check_frozen(relation regclass, t_ctid OUT tid) returns setof tid</function></term>
<listitem>
<para>
- Returns the TIDs of non-frozen tuples present in pages marked all-frozen
+ Returns the TIDs of non-frozen tuples stored in pages marked all-frozen
in the visibility map. If this function returns a non-empty set of
- TIDs, the database is corrupt.
+ TIDs, the visibility map is corrupt.
</para>
</listitem>
</varlistentry>
-
- <varlistentry>
- <term><function>pg_check_visible(regclass, t_ctid OUT tid) returns setof tid</function></term>
+
+ <varlistentry>
+ <term><function>pg_check_visible(relation regclass, t_ctid OUT tid) returns setof tid</function></term>
<listitem>
<para>
- Returns the TIDs of tuples which are not all-visible despite the fact
- that the pages which contain them are marked as all-visible in the
- visibility map. If this function returns a non-empty set of TIDs, the
- database is corrupt.
+ Returns the TIDs of non-all-visible tuples stored in pages marked
+ all-visible in the visibility map. If this function returns a non-empty
+ set of TIDs, the visibility map is corrupt.
</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><function>pg_truncate_visibility_map(regclass) returns void</function></term>
+ <term><function>pg_truncate_visibility_map(relation 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.
+ Truncates the visibility map for the given relation. This function is
+ useful if you believe that the visibility map for the relation is
+ corrupt and wish to force rebuilding 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. (Until that
+ is done, queries will treat the visibility map as containing all zeroes.)
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
- By default, these functions are not publicly executable.
+ By default, these functions are executable only by superusers.
</para>
</sect2>
projects / postgresql / commitdiff