sql - UNION 子查询的奇怪行为，为什么？

Question

下面两个查询返回相同的信息；但是，第一个需要 1.1 秒才能完成，第二个需要 0.06 秒。

此表有198,810 条记录，"Contacts"."ReqID" 已编入索引。

1. 为什么(看似)不那么复杂的查询会导致性能下降？
2. 为什么 UNION 子查询会加速这个过程？

*编辑*

我快速连续地运行了这两个查询，没有发现性能有任何变化。

---

查询 1

SELECT
    "Contacts"."ContactID",
    "Contacts"."ReqID"
FROM
    "Contacts"
WHERE
    "Contacts"."ReqID" = 2426;

*编辑*

解释分析

Index Scan using "Contacts_ReqID_idx" on "Contacts"  (cost=0.00..30.08 rows=11 width=78) (actual time=0.076..0.115 rows=14 loops=1)
  Index Cond: ("ReqID" = 2426)
Total runtime: 0.159 ms

1.1 秒返回14 条记录。

---

查询2

SELECT
    "T1"."ContactID",
    "T1"."ReqID"
FROM
    (
        SELECT
            "Contacts"."ContactID",
            "Contacts"."ReqID"
        FROM
            "Contacts"
        WHERE
            "Contacts"."ReqID" = 2426
        UNION
        SELECT
            "Contacts"."ContactID",
            "Contacts"."ReqID"
        FROM
            "Contacts"
        WHERE
            "Contacts"."ReqID" = 2426
    ) AS "T1"
ORDER BY
    "ReqID"

*编辑*

解释分析

Sort (cost=61.74..61.80 rows=22 width=100) (actual time=0.313..0.329 rows=14 loops=1)
  Sort Key: "Contacts"."ReqID"
  Sort Method: quicksort  Memory: 26kB
  ->  HashAggregate  (cost=60.81..61.03 rows=22 width=78) (actual time=0.266..0.285 rows=14 loops=1)
        ->  Append  (cost=0.00..60.37 rows=22 width=78) (actual time=0.063..0.201 rows=28 loops=1)
              ->  Index Scan using "Contacts_ReqID_idx" on "Contacts"  (cost=0.00..30.08 rows=11 width=78) (actual time=0.059..0.106 rows=14 loops=1)
                Index Cond: ("ReqID" = 2426)
              ->  Index Scan using "Contacts_ReqID_idx" on "Contacts"  (cost=0.00..30.08 rows=11 width=78) (actual time=0.006..0.024 rows=14 loops=1)
                Index Cond: ("ReqID" = 2426)
Total runtime: 0.410 ms

0.06 秒返回14 条记录。

Answer 1

事实证明 Navicat(我用来访问和管理我的数据库的软件)正在运行一些额外的代码，允许在运行简单的 SELECT * FROM [Table] 时就地编辑数据陈述。以下是我从支持代表那里收到的电子邮件。

---

Dear [Navicat User],

Thanks for your email. Please note that in order to get column information and allow update data afterwards, Navicat will run extra SQL when the query is a simple SELECT query. Using UNION, Navicat not allows user update any data in the query result.

...

Sincerely,
Mayho Ho
Navicat Support Center