如何在 Postgresql 中正确使用 FETCH FIRST？
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How to correctly use FETCH FIRST in Postgresql?
提问by Tyler Chong
I was trying to find out how to only select the first element of the SELECT query.
我试图找出如何只选择 SELECT 查询的第一个元素。
It seems alot of people use LIMIT 1 to only select the first one, but that doesn't seem like the best way to do it.
似乎很多人使用 LIMIT 1 只选择第一个，但这似乎不是最好的方法。
I was reading up on SELECT in the Postgresql docs, and it seems there is an option for a FETCH statement but I can't find any examples online, could someone explain to me how to correctly use it?
我正在阅读 Postgresql 文档中的 SELECT ，似乎有一个 FETCH 语句选项，但我在网上找不到任何示例，有人可以向我解释如何正确使用它吗？
回答by jmelesky
The following statements are equivalent:
以下语句是等效的：
SELECT * FROM foo LIMIT 10;
and
和
SELECT * FROM foo FETCH FIRST 10 ROWS ONLY;
ROWSis interchangeable with ROW, which makes fetching just 1 a little more grammatically consistent.
ROWS可与 互换ROW，这使得仅获取 1 在语法上更加一致。
FETCH FIRST X ROWS ONLYis part of the SQL standard, while, to my recollection, LIMITis not. LIMITis very popular, and much more terse, so it is also supported by postgres.
FETCH FIRST X ROWS ONLY是 SQL 标准的一部分，而据我所知，LIMIT不是。LIMIT非常流行，而且更加简洁，因此 postgres 也支持它。
Edited to add: The two statements are only syntactically different. They generate exactly the same plans:
编辑添加：这两个语句仅在语法上不同。它们生成完全相同的计划：
=# explain select * from foo fetch first 10 row only;
QUERY PLAN
-------------------------------------------------------------
Limit (cost=0.00..0.22 rows=10 width=68)
-> Seq Scan on foo (cost=0.00..18.50 rows=850 width=68)
=# explain select * from foo limit 10;
QUERY PLAN
-------------------------------------------------------------
Limit (cost=0.00..0.22 rows=10 width=68)
-> Seq Scan on foo (cost=0.00..18.50 rows=850 width=68)
