Second pass at improving LIKE/regex estimation in non-C locales. It turns

out that it's actually quite likely that a string that is an extension of
the given prefix will sort as larger than the "greater" string our previous
code created.  To provide some defense against that, do the comparisons
against a modified string instead of just the bare prefix.  We tack on
"Z", "z", "y", or "9", whichever is seen as largest in the current locale.
Testing suggests that this is sufficient at least for cases involving
ASCII data.

src/backend/utils/adt/selfuncs.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.238 2007/11/07 22:37:24 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.239 2007/11/09 20:10:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4586,7 +4586,10 @@ pattern_selectivity(Const *patt, Pattern_Type ptype)
 /*
  * Try to generate a string greater than the given string or any
  * string it is a prefix of.  If successful, return a palloc'd string
- * in the form of a Const pointer; else return NULL.
+ * in the form of a Const node; else return NULL.
+ *
+ * The caller must provide the appropriate "less than" comparison function
+ * for testing the strings.
  *
  * The key requirement here is that given a prefix string, say "foo",
  * we must be able to generate another string "fop" that is greater than
@@ -4595,11 +4598,13 @@ pattern_selectivity(Const *patt, Pattern_Type ptype)
  * that is not a bulletproof guarantee that an extension of the string might
  * not sort after it; an example is that "foo " is less than "foo!", but it
  * is not clear that a "dictionary" sort ordering will consider "foo!" less
- * than "foo bar".  Therefore, this function should be used only for
+ * than "foo bar".  CAUTION: Therefore, this function should be used only for
  * estimation purposes when working in a non-C locale.
  *
- * The caller must provide the appropriate "less than" comparison function
- * for testing the strings.
+ * To try to catch most cases where an extended string might otherwise sort
+ * before the result value, we determine which of the strings "Z", "z", "y",
+ * and "9" is seen as largest by the locale, and append that to the given
+ * prefix before trying to find a string that compares as larger.
  *
  * If we max out the righthand byte, truncate off the last character
  * and start incrementing the next.  For example, if "z" were the last
@@ -4615,13 +4620,22 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc)
 	Oid			datatype = str_const->consttype;
 	char	   *workstr;
 	int			len;
+	Datum		cmpstr;
+	text	   *cmptxt = NULL;
 
-	/* Get a modifiable copy of the string in C-string format */
+	/*
+	 * Get a modifiable copy of the prefix string in C-string format,
+	 * and set up the string we will compare to as a Datum.  In C locale
+	 * this can just be the given prefix string, otherwise we need to add
+	 * a suffix.  Types NAME and BYTEA sort bytewise so they don't need
+	 * a suffix either.
+	 */
 	if (datatype == NAMEOID)
 	{
 		workstr = DatumGetCString(DirectFunctionCall1(nameout,
 													  str_const->constvalue));
 		len = strlen(workstr);
+		cmpstr = str_const->constvalue;
 	}
 	else if (datatype == BYTEAOID)
 	{
@@ -4632,12 +4646,41 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc)
 		memcpy(workstr, VARDATA(bstr), len);
 		if ((Pointer) bstr != DatumGetPointer(str_const->constvalue))
 			pfree(bstr);
+		cmpstr = str_const->constvalue;
 	}
 	else
 	{
 		workstr = DatumGetCString(DirectFunctionCall1(textout,
 													  str_const->constvalue));
 		len = strlen(workstr);
+		if (lc_collate_is_c() || len == 0)
+			cmpstr = str_const->constvalue;
+		else
+		{
+			/* If first time through, determine the suffix to use */
+			static char suffixchar = 0;
+
+			if (!suffixchar)
+			{
+				char *best;
+
+				best = "Z";
+				if (varstr_cmp(best, 1, "z", 1) < 0)
+					best = "z";
+				if (varstr_cmp(best, 1, "y", 1) < 0)
+					best = "y";
+				if (varstr_cmp(best, 1, "9", 1) < 0)
+					best = "9";
+				suffixchar = *best;
+			}
+
+			/* And build the string to compare to */
+			cmptxt = (text *) palloc(VARHDRSZ + len + 1);
+			SET_VARSIZE(cmptxt, VARHDRSZ + len + 1);
+			memcpy(VARDATA(cmptxt), workstr, len);
+			*(VARDATA(cmptxt) + len) = suffixchar;
+			cmpstr = PointerGetDatum(cmptxt);
+		}
 	}
 
 	while (len > 0)
@@ -4665,10 +4708,12 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc)
 				workstr_const = string_to_bytea_const(workstr, len);
 
 			if (DatumGetBool(FunctionCall2(ltproc,
-										   str_const->constvalue,
+										   cmpstr,
 										   workstr_const->constvalue)))
 			{
-				/* Successfully made a string larger than the input */
+				/* Successfully made a string larger than cmpstr */
+				if (cmptxt)
+					pfree(cmptxt);
 				pfree(workstr);
 				return workstr_const;
 			}
@@ -4695,6 +4740,8 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc)
 	}
 
 	/* Failed... */
+	if (cmptxt)
+		pfree(cmptxt);
 	pfree(workstr);
 
 	return NULL;