How to improve the performance of the left external joiin

Hello:

I have a problem on the left outer join.
Here is the example:

SELECT A.N, GISELE
A.
LEFT OUTER JOIN B ON A.N = B.N

The main problem is that if the lines of A is 1000, and B is 10,000,000, this sql speed
still weak, is - is it possible to improve this. Thank you very much.

Hello

rp0428 wrote:
Of course - this will do:

SELECT N FROM A;

Hah - you got this question in an interview?

If you did it was a trick question.

It's more complicated than you think.
What you gave would be the exact equivalent

SELECT A.N    -- , B.N
FROM A
LEFT OUTER JOIN B ON A.N=B.N

However, for the second column in the output, we really need to look at table B.

I suspect that the outer join is the fastest way to get results. Make sure that n is indexed, and that statistics are up-to-date.
If you want to try another way, here is one:

SELECT     n
,     (
         SELECT  n
         FROM    b
         WHERE   n       = a.n
         AND         ROWNUM  = 1
     )         AS b_n
FROM    a
;

Published by: Frank Kulash, March 21, 2013 21:29
Added example scalar subquery.

Tags: Database

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