Avoid divide-by-zero in regex_selectivity() with long fixed prefix.

tglsfdc · tglsfdc · commit 0347470b31c1 · 2021-02-12T16:26:47.000-05:00
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
diff --git a/src/backend/utils/adt/like_support.c b/src/backend/utils/adt/like_support.c
@@ -1424,9 +1424,18 @@ 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);
