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Avoid divide-by-zero in regex_selectivity() with long fixed prefix.
Given a regex pattern with a very long fixed prefix (approaching 500 characters), the result of pow(FIXED_CHAR_SEL, fixed_prefix_len) can underflow to zero. Typically the preceding selectivity calculation would have underflowed as well, so that we compute 0/0 and get NaN. In released branches this leads to an assertion failure later on. That doesn't happen in HEAD, for reasons I've not explored yet, but it's surely still a bug. To fix, just skip the division when the pow() result is zero, so that we'll (most likely) return a zero selectivity estimate. In the edge cases where "sel" didn't yet underflow, perhaps this isn't desirable, but I'm not sure that the case is worth spending a lot of effort on. The results of regex_selectivity_sub() are barely worth the electrons they're written on anyway :-( Per report from Alexander Lakhin. Back-patch to all supported versions. Discussion: https://postgr.es/m/6de0a0c3-ada9-cd0c-3e4e-2fa9964b41e3@gmail.com
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@ -6482,9 +6482,18 @@ regex_selectivity(const char *patt, int pattlen, bool case_insensitive,
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sel *= FULL_WILDCARD_SEL;
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}
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/* If there's a fixed prefix, discount its selectivity */
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/*
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* If there's a fixed prefix, discount its selectivity. We have to be
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* careful here since a very long prefix could result in pow's result
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* underflowing to zero (in which case "sel" probably has as well).
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*/
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if (fixed_prefix_len > 0)
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sel /= pow(FIXED_CHAR_SEL, fixed_prefix_len);
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{
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double prefixsel = pow(FIXED_CHAR_SEL, fixed_prefix_len);
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if (prefixsel > 0.0)
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sel /= prefixsel;
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}
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/* Make sure result stays in range */
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CLAMP_PROBABILITY(sel);
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