Speed up summary truncate replacement logic

This was calling strlen on every iteration of the loop, causing
excessive runtime on deeply nested queries (e.g. hundreds of UNION ALL).

In passing, make explicit an assumption that the replacement string is as
long, or shorter, than the pattern we're matching against.

Author: lfittl

src/pg_query_summary_truncate.c

@@ -287,15 +287,23 @@ global_replace(char *str, char *pattern, char *replacement)
 {
 	size_t		plen = strlen(pattern);
 	size_t		rlen = strlen(replacement);
+	size_t		slen = strlen(str);
 
-	for (size_t i = 0; i < strlen(str); i++)
+	Assert(plen >= rlen);
+
+	for (size_t i = 0; i < slen; i++)
 	{
 		if (memcmp(str + i, pattern, plen) == 0)
 		{
-			size_t		len = strlen(str + i + plen);
-
 			memcpy(str + i, replacement, rlen);
-			memmove(str + i + rlen, str + i + plen, len + 1);
+
+			// Shorten the string if needed
+			if (plen > rlen)
+			{
+				size_t skip = i + plen;
+				memmove(str + i + rlen, str + skip, slen - skip + 1);
+				slen -= (plen - rlen);
+			}
 		}
 	}
 }