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:
    100%解决XP系统asp http500内部错误[转自XX博客]
    .net使用DotNetCharting控件生成报表统计图总结 (转地址)
    关于request取中文字符串变?的解决办法
    Delete删除大批量数据无响应的解决办法
    点击GridView模版列里的按钮取GridView当前被操作行的数据(转载)
    繁體字顯示問題
    寫博客過程中遇到的問題
    box model相關的API
    Buddhism常用術語
    靡不有初鮮克有終
  • 原文地址:https://www.cnblogs.com/gaojian/p/3140795.html
Copyright © 2011-2022 走看看