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When I designed the Bitmapset module, I set things up so that an empty Bitmapset could be represented either by a NULL pointer, or by an allocated object all of whose bits are zero. I've recently come to the conclusion that that was a bad idea and we should instead have a convention like the longstanding invariant for Lists, whereby an empty list is represented by NIL and nothing else. To do this, we need to fix bms_intersect, bms_difference, and a couple of other functions to check for having produced an empty result; but then we can replace bms_is_empty(a) by a simple "a == NULL" test. This is very likely a (marginal) win performance-wise, because we call bms_is_empty many more times than those other functions put together. However, the real reason to do it is that we have various places that have hand-implemented a rule about "this Bitmapset variable must be exactly NULL if empty", so that they can use checks-for-null in place of bms_is_empty calls in particularly hot code paths. That is a really fragile, mistake-prone way to do things, and I'm surprised that we've seldom been bitten by it. It's not well documented at all which variables have this property, so you can't readily tell which code might be violating those conventions. By making the convention universal, we can eliminate a subtle source of bugs. Patch by me; thanks to Nathan Bossart and Richard Guo for review. Discussion: https://postgr.es/m/1159933.1677621588@sss.pgh.pa.us
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