zoukankan      html  css  js  c++  java
  • Parameterized Path 的例子

    Improve the planner's ability to use nested loops with inner index scans (Tom Lane)
    
    The new "parameterized path" mechanism allows inner index scans to use values from relations that are more than one join level up from the scan. This can greatly improve performance in situations where semantic restrictions (such as outer joins) limit the allowed join orderings.

    http://www.postgresql.org/docs/current/static/release-9-2.html

    数据准备:

    postgres=# create table tst01(id integer);
    CREATE TABLE
    postgres=# 
    
    postgres=# insert into tst01 values(generate_series(1,100000));
    INSERT 0 100000
    postgres=# 
    
    postgres=# create index idx_tst01_id on tst01(id);
    CREATE INDEX
    postgres=# 

    运行:

    postgres=# prepare s(int) as select * from tst01 t where id < $1;
    PREPARE
    postgres=# explain execute s(2);
                                       QUERY PLAN                                    
    ---------------------------------------------------------------------------------
     Index Only Scan using idx_tst01_id on tst01 t  (cost=0.00..8.38 rows=1 width=4)
       Index Cond: (id < 2)
    (2 rows)
    
    postgres=# explain execute s(10000);
                                          QUERY PLAN                                       
    ---------------------------------------------------------------------------------------
     Index Only Scan using idx_tst01_id on tst01 t  (cost=0.00..337.64 rows=10130 width=4)
       Index Cond: (id < 10000)
    (2 rows)
    
    postgres=# explain execute s(1000000);
                              QUERY PLAN                           
    ---------------------------------------------------------------
     Seq Scan on tst01 t  (cost=0.00..1693.00 rows=100000 width=4)
       Filter: (id < 1000000)
    (2 rows)
    
    postgres=# explain execute s(100000);
                              QUERY PLAN                           
    ---------------------------------------------------------------
     Seq Scan on tst01 t  (cost=0.00..1693.00 rows=100000 width=4)
       Filter: (id < 100000)
    (2 rows)
    
    postgres=# 

    这是一个小例子,而且还是一个有些特殊的例子。

    对比一下在PostgreSQL9.1.0中的表现:

    postgres=# create table tst01(id integer);
    CREATE TABLE
    postgres=# insert into tst01 values(generate_series(1,100000));
    INSERT 0 100000
    postgres=# create index idx_tst01_id on tst01(id);
    CREATE INDEX
    postgres=# prepare s(int) as select * from tst01 t where id < $1;
    PREPARE
    postgres=# explain execute s(2);
                                       QUERY PLAN                                   
     
    --------------------------------------------------------------------------------
    -
     Bitmap Heap Scan on tst01 t  (cost=626.59..1486.25 rows=33333 width=4)
       Recheck Cond: (id < $1)
       ->  Bitmap Index Scan on idx_tst01_id  (cost=0.00..618.26 rows=33333 width=0)
             Index Cond: (id < $1)
    (4 rows)
    
    postgres=# explain execute s(10000);
                                       QUERY PLAN                                   
     
    --------------------------------------------------------------------------------
    -
     Bitmap Heap Scan on tst01 t  (cost=626.59..1486.25 rows=33333 width=4)
       Recheck Cond: (id < $1)
       ->  Bitmap Index Scan on idx_tst01_id  (cost=0.00..618.26 rows=33333 width=0)
             Index Cond: (id < $1)
    (4 rows)
    
    postgres=# 

    可以看到,在9.1里,是不区分状况,执行计划固定。

  • 相关阅读:
    我与酷派手机N900+的艰难之旅
    从ASPNET1.1迁移到了ASPNET2.0遇到ORA01000: 超出打开游标的最大数
    彩铃平台接口独立取得阶段性胜利
    中央音乐平台的MD5算法问题
    彩铃接口独立完毕
    'OraOLEDB.Oracle.1' provider is not registered on the local machine.错误的解决
    程序员的陷阱
    ASP NET页面下载程序
    从aspnet1.1升级到aspnet2.0的小问题
    彩铃平台接口的计费服务问题解决
  • 原文地址:https://www.cnblogs.com/gaojian/p/3140795.html
Copyright © 2011-2022 走看看