Tuesday, August 16, 2011

Left Outer Join...

When one writes complex queries usually there are lot of tables are joined, the query becomes little bit more involved when Left Outer Join or Right Outer Join is involved. In case the query is like a straight left outer join query , the one listed below:

Select Count(*)
From A
Left Join B
Where A.AID Is Null

It will always return 0 records if there is no null AID in table A. When a left outer join is involved with other tables which are connected by inner joins, care needs to taken while performing checks like Where A.AID Is Null. I had an issue where a particular query joins and left outer joins was working fine, one fine day after the data was loaded, the query was taking for forever to complete. I was able to find out that the underlying data in the tables had chnaged which caused the query to take a long time. On further analysis in the WHERE clause of the query there was a check for a column to be NULL and this was combined with a OR clause.
It was some thing like : Where A.AID Is Null OR (cond1 and cond2), this was causing the query to run forever. The fix for this was we re-organized the query into 2 parts, the first part of the query runs with a WHERE clause checking for null, the second query runs with the other conditions listed after the OR clause. This caused the query to speed up tremendously and gave us the desired results.

In short when having complex queries and left outer joins it is best to test the queries with different variations of data to make sure the query returns the desired results.

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