How to create fast database queries

Archive for June 19th, 2009

Ordering NULLs

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From Stack Overflow:

In our product we have a generic search engine, and trying to optimize the search performance.

A lot of the tables used in the queries allow NULL values.

Should we redesign our table to disallow NULL values for optimization or not?

NULL values surely affect some optimizer decisions in Oracle.

It's widely known that Oracle won't include a column into index if all key values are NULL.

This means that if we don't apply a filter which would imply non-NULL condition on the column (like, any comparison operator in the WHERE clause, or a comparison operation in the ON condition of a JOIN clause), the index is not guaranteed to return all values, and a FULL SCAN will be required on the table.

It is less evident that ORDER BY queries can be affected too.

Let's create a sample table and see how:
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Written by Quassnoi

June 19th, 2009 at 11:00 pm

Posted in Oracle