Oracle在评估rownum之前是否会获取所有行? [英] Does Oracle fetch all the rows before evaluating rownum?

问题描述

我有一个表，该表将具有数百万条记录.

I have a table that will have millions of records.

我想确保只要找到第一个匹配项，Oracle就会停止查找.

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

对查询运行一个解释计划，您会发现您听错了-一旦将行标识为Oracle，Oracle将立即应用rownum谓词.基于其余查询的结果集的一部分.

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.

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