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Re: Retrieved columns affects index usage
Posted by: KimSeong Loh
Date: February 13, 2007 06:37PM

The optimiser has to use some assumptions since it does not know the exact data in the table until it executes the statement. The 1/2 or 2/3 data is unkown to the optimiser. The only available information to the MySQL optimiser is the cardinality, number of distinct value in the index, and if I remember correctly, even this is not exactly accurate with InnoDB.

You can try force_index in special situation like this.

But, if it is going to read 1/3 of the data, a full table scan might be faster than index scan sometimes.

As for
SELECT AColumn FROM ATABLE WHERE NOT (AColumn='')
My guess, it only need to read the index without having to read the data itself, this is a covering index, so it may decide to read the index, and since '' is the smallest value in the column, it can actually make it AColumn>'' and use a range on the index, otherwise it could be "index"

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