在 SQL 中使用 WITH 子句的指南 [英] Guidance on using the WITH clause in SQL

问题描述

我了解如何将 WITH 子句用于递归查询 (!!)，但我无法理解其一般用途/功能.

I understand how to use the WITH clause for recursive queries (!!), but I'm having problems understanding its general use / power.

例如，以下查询更新一条记录，其 id 是通过使用通过时间戳返回第一条记录的 id 的子查询来确定的:

For example the following query updates one record whose id is determined by using a subquery returning the id of the first record by timestamp:

update global.prospect psp
set    status=status||'*'
where  psp.psp_id=(
           select  p2.psp_id
           from    global.prospect p2
           where   p2.status='new' or p2.status='reset'
           order   by p2.request_ts
           limit   1 )
returning psp.*;

这会是使用 WITH 包装器而不是相对难看的子查询的好选择吗?如果有，为什么?

Would this be a good candidate for using a WITH wrapper instead of the relatively ugly sub-query? If so, why?

推荐答案

如果可以并发写入访问涉及的表，则在以下查询中存在竞争条件.考虑:

If there can be concurrent write access to involved tables, there are race conditions in the following queries. Consider:

• Postgres 更新…限制 1

您的示例可以使用 CTE(公用表表达式)，但它不会为您提供子查询无法做到的任何事情:

Your example can use a CTE (common table expression), but it will give you nothing a subquery couldn't do:

WITH x AS (
   SELECT  psp_id
   FROM    global.prospect
   WHERE   status IN ('new', 'reset')
   ORDER   BY request_ts
   LIMIT   1
   )
UPDATE global.prospect psp
SET    status = status || '*'
FROM   x
WHERE  psp.psp_id = x.psp_id
RETURNING psp.*;

返回的行将是更新的版本.

如果您想将返回的行插入到另一个表中，这就是 WITH 子句变得必不可少的地方:

If you want to insert the returned row into another table, that's where a WITH clause becomes essential:

WITH x AS (
   SELECT  psp_id
   FROM    global.prospect
   WHERE   status IN ('new', 'reset')
   ORDER   BY request_ts
   LIMIT   1
   )
, y AS (
   UPDATE global.prospect psp
   SET    status = status || '*'
   FROM   x
   WHERE  psp.psp_id = x.psp_id
   RETURNING psp.*
   )
INSERT INTO z
SELECT *
FROM   y;

PostgreSQL 9.1 添加了使用 CTE 的数据修改查询.
WITH 查询 (CTE) 手册.

Data-modifying queries using CTEs were added with PostgreSQL 9.1.
The manual about WITH queries (CTEs).

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