Re: Access and Filter Predicate on same execution plan line

Mark, one of the basic rules of the database design is to put the most selective column first in a multi-column index. Putting a column with very few values as the first column of a multi-column index goes against the time tested design rules and is guaranteed to cause problems. I have always treated skip scan as a performance problem.

As for the 42 billions, I do agree that the answer to the question of the indexes, databases and everything is 42. I believe that's in one of the Jonathan's books, along with the SEP field of the tablespace headers. BTW, I wonder what will be the Oracle version at the end of the universe?

Regards

On 8/17/21 8:51 PM, Mark W. Farnham wrote:
> If your first column is just two values, say, either Y or N, then a skip scan filtering on the second in each of Y and N will be the good polar case of skip scan. (Well, actually all values equal would be even better, but that would also be a silly column to have and index).
>
> If you have 42 billion different values for the first column only a few of which have the desired value in the second column, that will be toward the polar bad case.

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Mladen Gogala
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