Re: qsort again (was Re: Strange Create Index behaviour)

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

Ron <rjpeace(at)earthlink(dot)net> writes:
> How are we choosing our pivots?

See qsort.c: it looks like median of nine equally spaced inputs (ie,
the 1/8th points of the initial input array, plus the end points),
implemented as two rounds of median-of-three choices.  With half of the
data inputs zero, it's not too improbable for two out of the three
samples to be zeroes in which case I think the med3 result will be zero
--- so choosing a pivot of zero is much more probable than one would
like, and doing so in many levels of recursion causes the problem.

I think.  I'm not too sure if the code isn't just being sloppy about the
case where many data values are equal to the pivot --- there's a special
case there to switch to insertion sort, and maybe that's getting invoked
too soon.  It'd be useful to get a line-level profile of the behavior of
this code in the slow cases...

			regards, tom lane