准备语句

    PREPARE <identifier> AS <select-query>

预处理语句使得能够创建一个可多次执行的 SQL 语句。与执行语句一起使用时，这通过消除每次执行语句时的语句编译步骤来节省处理器时间。

<标识符> 指定了预编译语句的名称。查询中的参数使用“？”占位符表示；例如：

    prepare lookup as select * from t where x=?;

除非显式进行类型转换，否则所有参数都被假定为字符串类型。例如，以下语句允许将整数值替换到查询字符串中：

    prepare lookup as select * from t where x=?::integer;

以下示例语句演示了如何定义和使用预编译语句：

    create table t(x integer primary key, y integer);
    prepare lookup as select * from t where x=?::integer;
            
    prepare lookup2 as select * from t where x=?::integer and y=?::integer;
            
    insert into t values (1,10);
    execute lookup(1);
    explain execute lookup(1);
    execute lookup2(1,10);
    explain execute lookup2(1,10);
    prepare selfjoin as select * from t as t1 left join t as t2 on t1.y=t2.y;
            
    execute selfjoin();
    explain execute selfjoin();
    execute selfjoin();