zoukankan      html  css  js  c++  java
  • 对PostgreSQL的prepared statement 的理解

    http://www.postgresql.org/docs/9.2/static/sql-prepare.html

    PREPARE creates a prepared statement. A prepared statement is a server-side object that can be used to optimize performance. When the PREPARE statement is executed, the specified statement is parsed, analyzed, and rewritten. When an EXECUTE command is subsequently issued, the prepared statement is planned and executed. This division of labor avoids repetitive parse analysis work, while allowing the execution plan to depend on the specific parameter values supplied.

    在psql窗口A进行试验:

    prepare 动作:

    postgres=# prepare test(int) AS
    postgres-# select * from customers c where c.cust_id = $1;
    PREPARE
    postgres=# 

    execute 动作:

    postgres=# execute test(3);
     cust_id | cust_name 
    ---------+-----------
           3 | Taylor
    (1 row)
    
    postgres=# 

    此时,看 pg_prepared_statements 的状况:

    postgres=# select * from pg_prepared_statements;
     name |                    statement                    |         prepare_time          | parameter_types | from_sql 
    ------+-------------------------------------------------+-------------------------------+-----------------+----------
     test | prepare test(int) AS                           +| 2013-06-14 15:58:22.796369+08 | {integer}       | t
          | select * from customers c where c.cust_id = $1; |                               |                 | 
    (1 row)
    
    postgres=# 

    在另外的一个窗口B,再接着进行试验,我就无法看到 pg_prepared_statements中有值了!

    postgres=# select * from pg_prepared_statements;
     name | statement | prepare_time | parameter_types | from_sql 
    ------+-----------+--------------+-----------------+----------
    (0 rows)
    
    postgres=# 

    甚至我在此窗口B中,再次执行 execute test(3) ,也会失败。

    postgres=# execute test(3);
    ERROR:  prepared statement "test" does not exist
    postgres=# 

    由此,可以得出结论:

    在PostgreSQL中,prepared statement 也是session 单位的。在为各个客户端服务的各个子进程之间,无法共享已经分析好的执行计划!

    如果我的应用程序里,拥有多个数据库连接,是无法共享prepared statement的。

  • 相关阅读:
    jQuery入门教程
    vue-lazyload 图片不更新
    Eggjs 设置跨域请求
    Vue.js错误: Maximum call stack size exceeded
    ubuntu nginx ssl 证书配置
    ubuntu 安装nginx, 出现 Unable to locate package
    nginx 判断移动端或者PC端 进入不同域名
    node.js 生成二维码
    Linux 配置ssh 免密码登录
    nodejs 从部署到域名访问
  • 原文地址:https://www.cnblogs.com/gaojian/p/3135805.html
Copyright © 2011-2022 走看看