If the high end of a range is below the low end of the range, there's
no values in it, so we can short-circuit that. If we don't, if the
low end is zero or higher, and the high end is below zero, we can
get very surprising behaviors, such as accepting values up to the
inverse of the high end. Add a test case for this and treat it the
same as a low range end above the field's maximum or a high end
below the field's minimum, returning an empty row immediately.
If a shard has never had any decimal values in it at all for a
field, the ValCount object returned has no DecimalVal, which could
cause a segfault if we don't check for it. Add a test case which
sporadically triggers that behavior (it's timing/luck related,
unfortunately), and then also fix it.
The computation of available shards is cheap, because realistically, virtually
no one has enough shards that the resulting bitmap is more than one container.
We don't try to fix this at the field/index levels because it's significantly
harder to do there, but I think the creation of these bitmaps is probably
the most expensive part, and switching the unions to union-in-place probably
reduces cost significantly.
Note that the bitmaps being unioned almost certainly have exactly one small
container in them.
We avoid using bitmapContains so often because that turns out to be expensive.
Also, if we produce more than runMaxSize runs, we're going to convert to
a bitmap container (or possibly an array container if there were over
2048 items, but they're all singletons), and we can streamline that by just
converting the source to bitmap and returning differenceBitmapBitmap, which
is faster in this case.
This appears to overall take about half as long in the workload I was
looking at.
