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The original coding of this function paid little attention to the possibility of overflow. There were actually three different hazards: 1. The range from bound1 to bound2 could exceed DBL_MAX, which on IEEE-compliant machines produces +Infinity in the subtraction. At best we'd lose all precision in the result, and at worst produce NaN due to dividing Inf/Inf. The range can't exceed twice DBL_MAX though, so we can fix this case by scaling all the inputs by 0.5. 2. We computed count * (operand - bound1), which is also at risk of float overflow, before dividing. Safer is to do the division first, producing a quotient that should be in [0,1), and even after allowing for roundoff error can't be outside [0,1]; then multiplying by count can't produce a result overflowing an int. (width_bucket_numeric does the multiplication first on the grounds that that improves accuracy of its result, but I don't think that a similar argument can be made in float arithmetic.) 3. If the division result does round to 1, and count is INT_MAX, the final addition of 1 would overflow an int. We took care of that in the operand >= bound2 case but did not consider that it could be possible in the main path. Fix that by moving the overflow-aware addition of 1 so it is done that way in all cases. The fix for point 2 creates a possibility that values very close to a bucket boundary will be rounded differently than they were before. I'm not troubled by that for HEAD, but it is an argument against putting this into the stable branches. Given that the cases being fixed here are fairly extreme and unlikely to be hit in normal use, it seems best not to back-patch. Mats Kindahl and Tom Lane Discussion: https://postgr.es/m/17876-61f280d1601f978d@postgresql.org
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