Remove precedence hacks no longer needed without postfix operators.

It's no longer necessary to assign explicit precedences to GENERATED,
NULL_P, PRESERVE, or STRIP_P.

Actually, we don't need to assign precedence to IDENT either; that was
really just there to govern the behavior of target_el's "a_expr IDENT"
production, which no longer ends with that terminal.  However, it seems
like a good idea to continue to do so, because it provides a reference
point for a precedence level that we can assign to other unreserved
keywords that lack a natural precedence level.

Research by Peter Eisentraut and John Naylor; comment rewrite by me.

Discussion: https://postgr.es/m/38ca86db-42ab-9b48-2902-337a0d6b8311@2ndquadrant.com

src/backend/parser/gram.y

@@ -744,22 +744,15 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %nonassoc	BETWEEN IN_P LIKE ILIKE SIMILAR NOT_LA
 %nonassoc	ESCAPE			/* ESCAPE must be just above LIKE/ILIKE/SIMILAR */
 /*
- * To support target_el without AS, we must give IDENT an explicit priority
+ * To support target_el without AS, it used to be necessary to assign IDENT an
- * between ESCAPE and Op.  We can safely assign the same priority to
+ * explicit precedence just less than Op.  While that's not really necessary
- * various unreserved keywords as needed to resolve ambiguities (this can't
+ * since we removed postfix operators, it's still helpful to do so because
- * have any bad effects since obviously the keywords will still behave the
+ * there are some other unreserved keywords that need precedence assignments.
- * same as if they weren't keywords).  We need to do this:
+ * If those keywords have the same precedence as IDENT then they clearly act
- * for PARTITION, RANGE, ROWS, GROUPS to support opt_existing_window_name;
+ * the same as non-keywords, reducing the risk of unwanted precedence effects.
- * for RANGE, ROWS, GROUPS so that they can follow a_expr without creating
- * postfix-operator problems;
- * for GENERATED so that it can follow b_expr;
- * and for NULL so that it can follow b_expr in ColQualList without creating
- * postfix-operator problems.
  *
- * To support CUBE and ROLLUP in GROUP BY without reserving them, we give them
+ * We need to do this for PARTITION, RANGE, ROWS, and GROUPS to support
- * an explicit priority lower than '(', so that a rule with CUBE '(' will shift
+ * opt_existing_window_name (see comment there).
- * rather than reducing a conflicting rule that takes CUBE as a function name.
- * Using the same precedence as IDENT seems right for the reasons given above.
  *
  * The frame_bound productions UNBOUNDED PRECEDING and UNBOUNDED FOLLOWING
  * are even messier: since UNBOUNDED is an unreserved keyword (per spec!),
@@ -769,9 +762,14 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
  * appear to cause UNBOUNDED to be treated differently from other unreserved
  * keywords anywhere else in the grammar, but it's definitely risky.  We can
  * blame any funny behavior of UNBOUNDED on the SQL standard, though.
+ *
+ * To support CUBE and ROLLUP in GROUP BY without reserving them, we give them
+ * an explicit priority lower than '(', so that a rule with CUBE '(' will shift
+ * rather than reducing a conflicting rule that takes CUBE as a function name.
+ * Using the same precedence as IDENT seems right for the reasons given above.
  */
-%nonassoc	UNBOUNDED		/* ideally should have same precedence as IDENT */
+%nonassoc	UNBOUNDED		/* ideally would have same precedence as IDENT */
-%nonassoc	IDENT GENERATED NULL_P PARTITION RANGE ROWS GROUPS PRECEDING FOLLOWING CUBE ROLLUP
+%nonassoc	IDENT PARTITION RANGE ROWS GROUPS PRECEDING FOLLOWING CUBE ROLLUP
 %left		Op OPERATOR		/* multi-character ops and user-defined operators */
 %left		'+' '-'
 %left		'*' '/' '%'
@@ -792,8 +790,6 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
  * left-associativity among the JOIN rules themselves.
  */
 %left		JOIN CROSS LEFT FULL RIGHT INNER_P NATURAL
-/* kluge to keep xml_whitespace_option from causing shift/reduce conflicts */
-%right		PRESERVE STRIP_P
 
 %%