Oracle 是否在评估 rownum 之前获取所有行?

Does Oracle fetch all the rows before evaluating rownum?(Oracle 是否在评估 rownum 之前获取所有行?)

问题描述

我有一个包含数百万条记录的表.

I have a table that will have millions of records.

我想确保 Oracle 只要找到第一个匹配项就停止查找.

I want to make sure Oracle stops looking as long as it finds its first match.

select * from table1
where table1.column1 = 'somevalue' 
  AND table2.column2 = 'somevalue' 
  AND rownum = 1

我听说 Oracle 将获取满足 column1 和 column2 条件的所有行，然后仅应用 rownum 过滤器来获取第一行，这违背了目的.

I heard about that Oracle will fetch ALL the rows that meet the criteria on column1 and column2 and then only apply the rownum filter to get the first row, which defeats the purpose.

推荐答案

在查询上运行一个解释计划，你会发现你听错了 -- Oracle 将在一行被识别为时应用 rownum 谓词结果集的一部分基于查询的其余部分.

Run an explain plan on the query, and you'll find that you heard wrong -- Oracle will apply the rownum predicate as soon as a row is identified as part of the result set based on the rest of the query.

这篇关于Oracle 是否在评估 rownum 之前获取所有行?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持编程学习网！