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commit    a28df6fa3041523f9a96cf0d3c3929f8bdc7c797
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Fri Feb 12 21:26:47 2021 +0000
First released in: 14.0, 9.6.22, 10.17, 11.12, 12.7, 13.3

    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


 src/backend/utils/adt/selfuncs.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 18f4bedb37a..17640b2328f 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -6482,9 +6482,18 @@ regex_selectivity(const char *patt, int pattlen, bool case_insensitive,
 		sel *= FULL_WILDCARD_SEL;
 	}
 
-	/* If there's a fixed prefix, discount its selectivity */
+	/*
+	 * If there's a fixed prefix, discount its selectivity.  We have to be
+	 * careful here since a very long prefix could result in pow's result
+	 * underflowing to zero (in which case "sel" probably has as well).
+	 */
 	if (fixed_prefix_len > 0)
-		sel /= pow(FIXED_CHAR_SEL, fixed_prefix_len);
+	{
+		double		prefixsel = pow(FIXED_CHAR_SEL, fixed_prefix_len);
+
+		if (prefixsel > 0.0)
+			sel /= prefixsel;
+	}
 
 	/* Make sure result stays in range */
 	CLAMP_PROBABILITY(sel);


[parent: db5b64b97829]