Question

我有一个（简化的）查询，如下：

SELECT DISTINCT this_.person_id AS y0_
  FROM coded_entity this_
 WHERE this_.code     = ?
   AND this_.tenant_id  = ?
   AND this_.person_id IN (SELECT person_id
                             FROM person that
                            WHERE that.date_voided IS NULL
                              AND that.ssn            = ?
                              AND that.tenant_id      = ? )

我希望能够强制Oracle始终首先执行最里面的子查询，因为我知道它总是会更具选择性。代码只有几个值，而ssn有更多的值。每个表都有相同的行数。

但是，在实际操作中，有时Oracle CBO会决定先执行coded_entity查询，导致时间慢得多。

有没有办法强制执行此操作而不将查询拆分为单独的调用？

Answer 1

我的第一个念头（无论是否有钥匙）都是这样想的：

SELECT DISTINCT this_.person_id AS y0_
  FROM coded_entity this_
 WHERE this_.code     = ?
   AND this_.tenant_id  = ?
   AND EXISTS (SELECT null
                 FROM person THAT_
                WHERE THAT_.date_voided IS NULL
                  AND THAT_.ssn            = ?
                  AND THAT_.tenant_id      = this_.tenant_id
                  AND THAT_.person_id      = this_.person_id)

但更好的方法（如果person_id是人的关键，并且每个人都有零个或多个编码实体）将是这样的：

SELECT this_.person_id AS y0_
  FROM person_ this_
 WHERE this_.ssn        = ?
   AND this_.tenant_id  = ?
   AND this_.date_voided is null
   AND EXISTS (SELECT null
                 FROM coded_entity THAT_
                WHERE THAT_.code = ?
                  AND THAT_.tenant_id      = this_.tenant_id
                  AND THAT_.person_id      = this_.person_id)

Answer 2

不需要子查询。只需INNER JOIN并过滤唯一记录。让优化器更好地理解它。

SELECT DISTINCT this_.person_id AS y0_
FROM coded_entity this_
INNER JOIN person that ON this_.person_id = that.person_id
    AND this_.tenant_id = that.tenant_id
    AND that.date_voided IS NULL
    AND that.ssn = ?
 WHERE this_.code     = ?
   AND this_.tenant_id  = ?

Answer 3

您始终可以重写脚本以使条件首先作为子选择应用，然后在其上应用其他条件，如下所示：

SELECT DISTINCT this_.person_id AS y0_
FROM (
SELECT *
  FROM coded_entity this_
 WHERE this_.person_id IN (SELECT person_id
                             FROM person that
                            WHERE that.date_voided IS NULL
                              AND that.ssn            = ?
                              AND that.tenant_id      = ? ) 
) sub
where this_.code     = ?
   AND this_.tenant_id  = ?

或者你可以用连接格式重写它：

SELECT DISTINCT this_.person_id AS y0_
  FROM coded_entity this_
  JOIN person that
    ON that.person_id = this_person_id
    AND that.date_voided IS NULL
    AND that.ssn            = ?
    AND that.tenant_id      = ?
 WHERE this_.code     = ?
   AND this_.tenant_id  = ?

或者我认为肯定会有效的是只要在两个表上正确收集统计数据，然后CBO就会正确决定。

强制SQL子查询首先运行

3 个答案: