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Change hash indexes to store only the hash code rather than the whole indexed

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value.  This means that hash index lookups are always lossy and have to be
rechecked when the heap is visited; however, the gain in index compactness
outweighs this when the indexed values are wide.  Also, we only need to
perform datatype comparisons when the hash codes match exactly, rather than
for every entry in the hash bucket; so it could also win for datatypes that
have expensive comparison functions.  A small additional win is gained by
keeping hash index pages sorted by hash code and using binary search to reduce
the number of index tuples we have to look at.

Xiao Meng

This commit also incorporates Zdenek Kotala's patch to isolate hash metapages
and hash bitmaps a bit better from the page header datastructures.
This commit is contained in:
Tom Lane
2008-09-15 18:43:41 +00:00
parent 440b3384b0
commit 4adc2f72a4
13 changed files with 313 additions and 129 deletions
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11
doc/src/sgml/catalogs.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.173 2008/09/10 18:09:19 alvherre Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.174 2008/09/15 18:43:41 tgl Exp $ -->
<!--
Documentation of the system catalogs, directed toward PostgreSQL developers
-->
@ -451,6 +451,13 @@
<entry>Can an index of this type be clustered on?</entry>
</row>
<row>
<entry><structfield>amkeytype</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal><link linkend="catalog-pg-type"><structname>pg_type</structname></link>.oid</literal></entry>
<entry>Type of data stored in index, or zero if not a fixed type</entry>
</row>
<row>
<entry><structfield>aminsert</structfield></entry>
<entry><type>regproc</type></entry>
@ -6424,7 +6431,7 @@
<row>
<entry><structfield>sourceline</structfield></entry>
<entry><type>text</type></entry>
<entry>Line number within the sourcefile the current value was set
<entry>Line number within the sourcefile the current value was set
from (NULL for values set in sources other than configuration files)
</entry>
</row>