Improve match_special_index_operator() to recognize that LIKE with an

exact-match pattern (no wildcard) can be index-optimized in some cases where a
prefix-match pattern cannot; specifically, since the required index clause is
simple equality, it works for regular text/varchar indexes even when the
locale is not C.  I'm not sure how often this case really comes up, but since
it requires hardly any additional work to handle it, we might as well get it
right.  Motivated by a discussion on the JDBC list.

src/backend/optimizer/path/indxpath.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.237 2009/03/05 23:06:45 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.238 2009/03/11 03:32:22 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2138,6 +2138,7 @@ match_special_index_operator(Expr *clause, Oid opfamily,
 	Const	   *patt;
 	Const	   *prefix = NULL;
 	Const	   *rest = NULL;
+	Pattern_Prefix_Status pstatus = Pattern_Prefix_None;
 
 	/*
 	 * Currently, all known special operators require the indexkey on the
@@ -2163,37 +2164,42 @@ match_special_index_operator(Expr *clause, Oid opfamily,
 		case OID_BPCHAR_LIKE_OP:
 		case OID_NAME_LIKE_OP:
 			/* the right-hand const is type text for all of these */
-			isIndexable = pattern_fixed_prefix(patt, Pattern_Type_Like,
-									  &prefix, &rest) != Pattern_Prefix_None;
+			pstatus = pattern_fixed_prefix(patt, Pattern_Type_Like,
+										   &prefix, &rest);
+			isIndexable = (pstatus != Pattern_Prefix_None);
 			break;
 
 		case OID_BYTEA_LIKE_OP:
-			isIndexable = pattern_fixed_prefix(patt, Pattern_Type_Like,
-									  &prefix, &rest) != Pattern_Prefix_None;
+			pstatus = pattern_fixed_prefix(patt, Pattern_Type_Like,
+										   &prefix, &rest);
+			isIndexable = (pstatus != Pattern_Prefix_None);
 			break;
 
 		case OID_TEXT_ICLIKE_OP:
 		case OID_BPCHAR_ICLIKE_OP:
 		case OID_NAME_ICLIKE_OP:
 			/* the right-hand const is type text for all of these */
-			isIndexable = pattern_fixed_prefix(patt, Pattern_Type_Like_IC,
-									  &prefix, &rest) != Pattern_Prefix_None;
+			pstatus = pattern_fixed_prefix(patt, Pattern_Type_Like_IC,
+										   &prefix, &rest);
+			isIndexable = (pstatus != Pattern_Prefix_None);
 			break;
 
 		case OID_TEXT_REGEXEQ_OP:
 		case OID_BPCHAR_REGEXEQ_OP:
 		case OID_NAME_REGEXEQ_OP:
 			/* the right-hand const is type text for all of these */
-			isIndexable = pattern_fixed_prefix(patt, Pattern_Type_Regex,
-									  &prefix, &rest) != Pattern_Prefix_None;
+			pstatus = pattern_fixed_prefix(patt, Pattern_Type_Regex,
+										   &prefix, &rest);
+			isIndexable = (pstatus != Pattern_Prefix_None);
 			break;
 
 		case OID_TEXT_ICREGEXEQ_OP:
 		case OID_BPCHAR_ICREGEXEQ_OP:
 		case OID_NAME_ICREGEXEQ_OP:
 			/* the right-hand const is type text for all of these */
-			isIndexable = pattern_fixed_prefix(patt, Pattern_Type_Regex_IC,
-									  &prefix, &rest) != Pattern_Prefix_None;
+			pstatus = pattern_fixed_prefix(patt, Pattern_Type_Regex_IC,
+										   &prefix, &rest);
+			isIndexable = (pstatus != Pattern_Prefix_None);
 			break;
 
 		case OID_INET_SUB_OP:
@@ -2217,9 +2223,17 @@ match_special_index_operator(Expr *clause, Oid opfamily,
 	 * want to apply.  (A hash index, for example, will not support ">=".)
 	 * Currently, only btree supports the operators we need.
 	 *
+	 * Note: actually, in the Pattern_Prefix_Exact case, we only need "="
+	 * so a hash index would work.  Currently it doesn't seem worth checking
+	 * for that, however.
+	 *
 	 * We insist on the opfamily being the specific one we expect, else we'd
 	 * do the wrong thing if someone were to make a reverse-sort opfamily with
 	 * the same operators.
+	 *
+	 * The non-pattern opclasses will not sort the way we need in most non-C
+	 * locales.  We can use such an index anyway for an exact match (simple
+	 * equality), but not for prefix-match cases.
 	 */
 	switch (expr_op)
 	{
@@ -2229,7 +2243,8 @@ match_special_index_operator(Expr *clause, Oid opfamily,
 		case OID_TEXT_ICREGEXEQ_OP:
 			isIndexable =
 				(opfamily == TEXT_PATTERN_BTREE_FAM_OID) ||
-				(opfamily == TEXT_BTREE_FAM_OID && lc_collate_is_c());
+				(opfamily == TEXT_BTREE_FAM_OID &&
+				 (pstatus == Pattern_Prefix_Exact || lc_collate_is_c()));
 			break;
 
 		case OID_BPCHAR_LIKE_OP:
@@ -2238,7 +2253,8 @@ match_special_index_operator(Expr *clause, Oid opfamily,
 		case OID_BPCHAR_ICREGEXEQ_OP:
 			isIndexable =
 				(opfamily == BPCHAR_PATTERN_BTREE_FAM_OID) ||
-				(opfamily == BPCHAR_BTREE_FAM_OID && lc_collate_is_c());
+				(opfamily == BPCHAR_BTREE_FAM_OID &&
+				 (pstatus == Pattern_Prefix_Exact || lc_collate_is_c()));
 			break;
 
 		case OID_NAME_LIKE_OP: