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About Tuning the SQL clause!

原创 Oracle 作者:YoungEric 时间:2007-10-11 17:07:13 0 删除 编辑

I don't think I konw how to tuning the SQL clause,and the following is a example ,and in the future ,I will add more!

[@more@]

--Before we see the example,I will first declare sth about the
--table A and B;
--A.column1 don't have a index
--B.column2 is the primark key of table B
--1
SELECT *
FROM A,B
WHERE A.column1=B.column2
--2
SELECT *
FROM A,B
WHERE B.column2=A.column1
--3
SELECT *
FROM A,B
WHERE SUBSTR(B.column2,1,2)=A.column1
--Frankly speaking ,I am nou quite sure about the method to
--see the time for executing the SQL clause;But I know the answer
--The answer is the second,for B.COLUMN2 is a primary key ,and it has a
--index,so it should be put to the left of "=";
--For the third one ,If we use a function on a index column , and it will not
--be used for tuniong.

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