<!-- $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>

</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>

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>

-&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>

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)

-&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>