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pg_visibility: Add pg_truncate_visibility_map function.

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This requires some core changes as well so that we can properly
WAL-log the truncation.  Specifically, it changes the format of the
XLOG_SMGR_TRUNCATE WAL record, so bump XLOG_PAGE_MAGIC.

Patch by me, reviewed but not fully endorsed by Andres Freund.
This commit is contained in:
Robert Haas
2016-06-17 17:37:30 -04:00
parent 54f5c5150f
commit 71d05a2c7b
8 changed files with 133 additions and 13 deletions
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30
doc/src/sgml/pgvisibility.sgml
View File

@@ -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>