Oracle SQL全外部联接 [英] Oracle SQL full outer join

问题描述

我使用FULL OUTER JOIN连接了2个表，这花了6分钟来运行并给出输出.

I was joining 2 tables using FULL OUTER JOIN which took 6 minutes to run and give the output.

SELECT * 
FROM tab1 FULL OUTER JOIN tab2
ON tab1.id = tab2.id
;

我使用LEFT OUTER JOIN和RIGHT OUTER JOIN的UNION做了同样的事情.只花了15秒

I did the same thing using UNION of LEFT OUTER JOIN and RIGHT OUTER JOIN. This took only 15 seconds

SELECT *
FROM tab1, tab2
WHERE tab1.id (+) = tab2.id

UNION 

SELECT *
FROM tab1, tab2
WHERE tab1.id  = tab2.id (+)
;

有人知道为什么会这样吗?

Does anyone know why this is happening?

推荐答案

两个表中可能都有很多行，共有几行，并且所讨论的列上没有索引.

You probably have a lot of rows in both tables, few rows in common, and no index on the columns in question.

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