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Commitf551348

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Clarify example of planner cost computation, per a suggestion from
James Shaw. Also update a couple of examples to reflect 8.3's improvedplan-printing code.
1 parent3e17ef1 commitf551348

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‎doc/src/sgml/perform.sgml

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