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Commitae4867e

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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 500characters), the result of pow(FIXED_CHAR_SEL, fixed_prefix_len) canunderflow to zero. Typically the preceding selectivity calculationwould 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, sothat we'll (most likely) return a zero selectivity estimate. Inthe edge cases where "sel" didn't yet underflow, perhaps thisisn't desirable, but I'm not sure that the case is worth spendinga lot of effort on. The results of regex_selectivity_sub() arebarely 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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‎src/backend/utils/adt/like_support.c

Lines changed: 11 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1442,9 +1442,18 @@ 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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doubleprefixsel=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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