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Change planner to use the current true disk file size as its estimate of

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a relation's number of blocks, rather than the possibly-obsolete value
in pg_class.relpages.  Scale the value in pg_class.reltuples correspondingly
to arrive at a hopefully more accurate number of rows.  When pg_class
contains 0/0, estimate a tuple width from the column datatypes and divide
that into current file size to estimate number of rows.  This improved
methodology allows us to jettison the ancient hacks that put bogus default
values into pg_class when a table is first created.  Also, per a suggestion
from Simon, make VACUUM (but not VACUUM FULL or ANALYZE) adjust the value
it puts into pg_class.reltuples to try to represent the mean tuple density
instead of the minimal density that actually prevails just after VACUUM.
These changes alter the plans selected for certain regression tests, so
update the expected files accordingly.  (I removed join_1.out because
it's not clear if it still applies; we can add back any variant versions
as they are shown to be needed.)
This commit is contained in:
Tom Lane
2004-12-01 19:00:56 +00:00
parent b7bcea64ec
commit 5374d097de
18 changed files with 1219 additions and 3170 deletions
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15
doc/src/sgml/perform.sgml
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@@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.47 2004/11/15 06:32:14 neilc Exp $
$PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.48 2004/12/01 19:00:27 tgl Exp $
-->
<chapter id="performance-tips">
@@ -389,14 +389,15 @@ SELECT relname, relkind, reltuples, relpages FROM pg_class WHERE relname LIKE 't
<para>
For efficiency reasons, <structfield>reltuples</structfield>
and <structfield>relpages</structfield> are not updated on-the-fly,
and so they usually contain only approximate values (which is good
enough for the planner's purposes). They are initialized with dummy
values (presently 1000 and 10 respectively) when a table is created.
They are updated by certain commands, presently <command>VACUUM</>,
<command>ANALYZE</>, and <command>CREATE INDEX</>. A stand-alone
and so they usually contain somewhat out-of-date values.
They are updated by <command>VACUUM</>, <command>ANALYZE</>, and a
few DDL commands such as <command>CREATE INDEX</>. A stand-alone
<command>ANALYZE</>, that is one not part of <command>VACUUM</>,
generates an approximate <structfield>reltuples</structfield> value
since it does not read every row of the table.
since it does not read every row of the table. The planner
will scale the values it finds in <structname>pg_class</structname>
to match the current physical table size, thus obtaining a closer
approximation.
</para>
<indexterm>