Bruce Momjian <bruce(at)momjian(dot)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