On Fri, 2011-10-07 at 12:54 +0400, Alexander Korotkov wrote:
> The first thing caught my eye in existing GiST code is idea of
> subtype_float. float8 has limited precision and can't respresent, for
> example, varlena values good enough. Even if we have large int8 value
> we can loose lower bits, but data distribution can be so that these
> bits are valuable. Wouldn't it better to have function like
> subtype_diff_float which returns difference between two values of
> subtype as an float? Using of such function could make penalty more
> sensible to even small difference between values, and accordingly more
> relevant.
The reason I did it that way is for unbounded ranges. With
subtype_diff_float, it's difficult for the GiST code to differentiate
between [10,) and [100000,), because infinity minus anything is
infinity. But when inserting the range [100,200), the penalty for the
first one should be zero and the second one should have some positive
penalty, right?
Maybe we can still use subtype_diff_float by calling it on various pairs
of bounds to come up with a reasonable cost?
I'm open to suggestion. I'd just like to make sure that unbounded ranges
are a part of the consideration.
Regards,
Jeff Davis
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