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Clarify example of planner cost computation, per a suggestion from

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James Shaw.  Also update a couple of examples to reflect 8.3's improved
plan-printing code.
This commit is contained in:
Tom Lane 2007-10-22 21:34:33 +00:00
parent 3e17ef1cfa
commit f551348417
1 changed files with 11 additions and 9 deletions
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20
doc/src/sgml/perform.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.65 2007/09/26 22:36:30 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.66 2007/10/22 21:34:33 tgl Exp $ -->
<chapter id="performance-tips">
<title>Performance Tips</title>
@ -164,10 +164,11 @@ SELECT relpages, reltuples FROM pg_class WHERE relname = 'tenk1';
</programlisting>
you will find out that <classname>tenk1</classname> has 358 disk
pages and 10000 rows. So the cost is estimated at 358 page
reads, costing <xref linkend="guc-seq-page-cost"> apiece (1.0 by
default), plus 10000 * <xref linkend="guc-cpu-tuple-cost"> which is
0.01 by default.
pages and 10000 rows. The estimated cost is (disk pages read *
<xref linkend="guc-seq-page-cost">) + (rows scanned *
<xref linkend="guc-cpu-tuple-cost">). By default,
<varname>seq_page_cost</> is 1.0 and <varname>cpu_tuple_cost</> is 0.01.
So the estimated cost is (358 * 1.0) + (10000 * 0.01) = 458.
</para>
<para>
@ -189,7 +190,8 @@ EXPLAIN SELECT * FROM tenk1 WHERE unique1 &lt; 7000;
The estimate of output rows has gone down because of the <literal>WHERE</>
clause.
However, the scan will still have to visit all 10000 rows, so the cost
hasn't decreased; in fact it has gone up a bit to reflect the extra CPU
hasn't decreased; in fact it has gone up a bit (by 10000 * <xref
linkend="guc-cpu-operator-cost">, to be exact) to reflect the extra CPU
time spent checking the <literal>WHERE</> condition.
</para>
@ -310,7 +312,7 @@ EXPLAIN SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 &lt; 100 AND t1.unique
-&gt; Bitmap Index Scan on tenk1_unique1 (cost=0.00..2.37 rows=106 width=0)
Index Cond: (unique1 &lt; 100)
-&gt; Index Scan using tenk2_unique2 on tenk2 t2 (cost=0.00..3.01 rows=1 width=244)
Index Cond: ("outer".unique2 = t2.unique2)
Index Cond: (t2.unique2 = t1.unique2)
</programlisting>
</para>
@ -356,7 +358,7 @@ EXPLAIN SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 &lt; 100 AND t1.unique
QUERY PLAN
------------------------------------------------------------------------------------------
Hash Join (cost=232.61..741.67 rows=106 width=488)
Hash Cond: ("outer".unique2 = "inner".unique2)
Hash Cond: (t2.unique2 = t1.unique2)
-&gt; Seq Scan on tenk2 t2 (cost=0.00..458.00 rows=10000 width=244)
-&gt; Hash (cost=232.35..232.35 rows=106 width=244)
-&gt; Bitmap Heap Scan on tenk1 t1 (cost=2.37..232.35 rows=106 width=244)
@ -395,7 +397,7 @@ EXPLAIN ANALYZE SELECT * FROM tenk1 t1, tenk2 t2 WHERE t1.unique1 &lt; 100 AND t
-&gt; Bitmap Index Scan on tenk1_unique1 (cost=0.00..2.37 rows=106 width=0) (actual time=0.546..0.546 rows=100 loops=1)
Index Cond: (unique1 &lt; 100)
-&gt; Index Scan using tenk2_unique2 on tenk2 t2 (cost=0.00..3.01 rows=1 width=244) (actual time=0.067..0.078 rows=1 loops=100)
Index Cond: ("outer".unique2 = t2.unique2)
Index Cond: (t2.unique2 = t1.unique2)
Total runtime: 14.452 ms
</screen>