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Add pg_visibility contrib module.

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This lets you examine the visibility map as well as page-level
visibility information.  I initially wrote it as a debugging aid,
but was encouraged to polish it for commit.

Patch by me, reviewed by Masahiko Sawada.

Discussion: 56D77803.6080503@BlueTreble.com
This commit is contained in:
Robert Haas
2016-03-08 08:38:50 -05:00
parent 6f56b41ac0
commit ba0a198fb1
9 changed files with 544 additions and 0 deletions
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1
doc/src/sgml/contrib.sgml
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@ -132,6 +132,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
&pgstatstatements;
&pgstattuple;
&pgtrgm;
&pgvisibility;
&postgres-fdw;
&seg;
&sepgsql;

1
doc/src/sgml/filelist.sgml
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@ -136,6 +136,7 @@
<!ENTITY pgstatstatements SYSTEM "pgstatstatements.sgml">
<!ENTITY pgstattuple SYSTEM "pgstattuple.sgml">
<!ENTITY pgtrgm SYSTEM "pgtrgm.sgml">
<!ENTITY pgvisibility SYSTEM "pgvisibility.sgml">
<!ENTITY postgres-fdw SYSTEM "postgres-fdw.sgml">
<!ENTITY seg SYSTEM "seg.sgml">
<!ENTITY contrib-spi SYSTEM "contrib-spi.sgml">

110
doc/src/sgml/pgvisibility.sgml Normal file
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@ -0,0 +1,110 @@
<!-- doc/src/sgml/pgvisibility.sgml -->
<sect1 id="pgvisibility" xreflabel="pg_visibility">
<title>pg_visibility</title>
<indexterm zone="pgvisibility">
<primary>pg_visibility</primary>
</indexterm>
<para>
The <filename>pg_visibility</> module provides a means for examining the
visibility map (VM) and page-level visibility information.
</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
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
same meaning as the all-visible bit in the visibility map, but is stored
within the data page itself rather than a separate data tructure. 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.
</para>
<para>
Functions which display information about <literal>PG_ALL_VISIBLE</>
are much more costly than those which only consult the visibility map,
because they must read the relation's data blocks rather than only the
(much smaller) visibility map.
</para>
<sect2>
<title>Functions</title>
<variablelist>
<varlistentry>
<term><function>pg_visibility_map(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
the given block of the given relation.
</para>
</listitem>
</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>
<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_VISIBILE</> bit for that block.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_visibility_map(regclass, blkno OUT 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
each block 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 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>pg_visibility_map_summary(regclass, all_visible OUT bigint, all_frozen OUT bigint) returns record</function></term>
<listitem>
<para>
Returns the number of all-visible pages and the number of all-frozen
pages in the relation according to the visibility map.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
By default, these functions are not publicly executable.
</para>
</sect2>
<sect2>
<title>Author</title>
<para>
Robert Haas <email>rhaas@postgresql.org</email>
</para>
</sect2>
</sect1>

5
doc/src/sgml/storage.sgml
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@ -648,6 +648,11 @@ might not be true. Visibility map bits are only set by vacuum, but are
cleared by any data-modifying operations on a page.
</para>
<para>
The <xref linkend="pgvisibility"> module can be used to examine the
information stored in the visibility map.
</para>
</sect1>
<sect1 id="storage-init">