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Improve handling of prune/no-prune decisions by storing a page's oldest

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unpruned XMAX in its header.  At the cost of 4 bytes per page, this keeps us
from performing heap_page_prune when there's no chance of pruning anything.
Seems to be necessary per Heikki's preliminary performance testing.
This commit is contained in:
Tom Lane
2007-09-21 21:25:42 +00:00
parent 386a5d4268
commit cc59049daf
10 changed files with 92 additions and 59 deletions
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16
doc/src/sgml/storage.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.18 2007/06/03 17:05:53 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.19 2007/09/21 21:25:42 tgl Exp $ -->
<chapter id="storage">
@ -416,7 +416,7 @@ Item
<row>
<entry>PageHeaderData</entry>
<entry>20 bytes long. Contains general information about the page, including
<entry>24 bytes long. Contains general information about the page, including
free space pointers.</entry>
</row>
@ -449,7 +449,7 @@ data. Empty in ordinary tables.</entry>
<para>
The first 20 bytes of each page consists of a page header
The first 24 bytes of each page consists of a page header
(PageHeaderData). Its format is detailed in <xref
linkend="pageheaderdata-table">. The first two fields track the most
recent WAL entry related to this page. Next is a 2-byte field
@ -459,7 +459,7 @@ data. Empty in ordinary tables.</entry>
from the page start to the start
of unallocated space, to the end of unallocated space, and to the start of
the special space.
The last 2 bytes of the page header,
The next 2 bytes of the page header,
<structfield>pd_pagesize_version</structfield>, store both the page size
and a version indicator. Beginning with
<productname>PostgreSQL</productname> 8.3 the version number is 4;
@ -471,6 +471,8 @@ data. Empty in ordinary tables.</entry>
versions, but the layout of heap row headers has.) The page size
is basically only present as a cross-check; there is no support for having
more than one page size in an installation.
The last field is a hint that shows whether pruning the page is likely
to be profitable: it tracks the oldest un-pruned XMAX on the page.
</para>
@ -530,6 +532,12 @@ data. Empty in ordinary tables.</entry>
<entry>2 bytes</entry>
<entry>Page size and layout version number information</entry>
</row>
<row>
<entry>pd_prune_xid</entry>
<entry>TransactionId</entry>
<entry>4 bytes</entry>
<entry>Oldest unpruned XMAX on page, or zero if none</entry>
</row>
</tbody>
</tgroup>
</table>