Re: Use of index for 50% column restriction

Bruce Momjian <bruce@momjian.us> writes:
> As part of my research on the parsing/planning behavior of PREPARE, I
> found a surprising behavior --- a WHERE clause that is 50% restrictive
> is using an index.  I thought only <10% restrictions used indexes.

There's no such hard-and-fast rule.  The cost estimate break point depends
greatly on the index order correlation (which is 100% in your example),
as well as some other factors like the index size versus
effective_cache_size.

For randomly-ordered data I believe the cutover is actually well below 10%.
        regards, tom lane