Question

我是数据库算法的新手，不知道它是如何在内部工作的。

因此，我尝试编写以下两个查询来比较性能：

SELECT * 
FROM (
    SELECT *
    FROM account
    WHERE id < 20000
) as dpa_r
FULL OUTER JOIN (
    SELECT *
    FROM transactions
    WHERE id < 20000
) as pet_r
ON SOME_CONDITION
FULL OUTER JOIN (
    SELECT *
    FROM profit
    WHERE id < 20000
) as dp_r
ON SOME_CONDITION

OFFSET 200000 LIMIT 20

查询执行时间= 940ms。

我重写了如下查询：

SELECT * 
FROM (
    SELECT *
    FROM account
) as dpa_r
FULL OUTER JOIN (
    SELECT *
    FROM transactions
) as pet_r
ON SOME_CONDITION
FULL OUTER JOIN (
    SELECT *
    FROM profit
) as dp_r
ON SOME_CONDITION
WHERE dp_r.id < 20000 AND dpa_r.id < 20000 AND pet_r.id < 20000
OFFSET 200000 LIMIT 20

查询执行时间为17321ms。

为什么时间差异如此之大？

Answer 1

您有两个查询给出不同的结果，其中一个是外部联接查询，但另一个是内部联接查询。可能是你获得不同执行时间的原因。

对于WHERE子句中的列条件，外部联接作为常规内部联接执行：

SQL>create table t1 (c1 int, c2 char(10));
SQL>create table t2 (c1 int, c2 char(10));
SQL>insert into t1 values (1,'One');
SQL>insert into t1 values (2,'Two');
SQL>insert into t2 values (1,'One');
SQL>insert into t2 values (5,'Five');
SQL>select * from t1 left join t2 on t1.c1 = t2.c1
SQL&where t1.c1 < 10 and t2.c1 < 10;
         c1 c2                  c1 c2
=========== ========== =========== ==========
          1 One                  1 One

                  1 row found

SQL>select * from t1 left join t2 on t1.c1 = t2.c1 
SQL&  and t1.c1 < 10 and t2.c1 < 10;
         c1 c2                  c1 c2
=========== ========== =========== ==========
          1 One                  1 One
          2 Two                  - -

                  2 rows found

当这些条件转移到ON条款时，你会得到真实的OUTER JOIN行为！

为什么加入受限制的表比限制连接表更快？

1 个答案: