diff options
| -rw-r--r-- | doc/src/sgml/perform.sgml | 20 |
1 files changed, 11 insertions, 9 deletions
diff --git a/doc/src/sgml/perform.sgml b/doc/src/sgml/perform.sgml index df90f37438b..1d3b8484f5e 100644 --- a/doc/src/sgml/perform.sgml +++ b/doc/src/sgml/perform.sgml @@ -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 < 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 < 100 AND t1.unique -> Bitmap Index Scan on tenk1_unique1 (cost=0.00..2.37 rows=106 width=0) Index Cond: (unique1 < 100) -> 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 < 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) -> Seq Scan on tenk2 t2 (cost=0.00..458.00 rows=10000 width=244) -> Hash (cost=232.35..232.35 rows=106 width=244) -> 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 < 100 AND t -> 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 < 100) -> 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> |