Fix for FETCH FIRST syntax problems

Per bug #15200, our support for sql2008 FETCH FIRST syntax is incomplete
to the extent that it should be regarded as a bug; the spec quite
clearly allows parameters/host variables in addition to constants, but
we don't.

Attached is a draft patch for fixing that, which additionally fixes the
ugly wart that OFFSET <x> ROW/ROWS and FETCH FIRST [<x>] ROW/ROWS ONLY
had very different productions for <x>; both now accept c_expr there.

Shift/reduce conflict is avoided by taking advantage of the fact that
ONLY is a fully reserved word.

I've checked that this change would not make it any harder to add
(post-2008 features) WITH TIES or PERCENT in the event that someone
wants to do that.

I think a case can be made that this should be backpatched; thoughts?

(While I can't find any visible change for existing working queries, one
change that does occur is that FETCH FIRST -1 ROWS ONLY now returns a
different error message; but that was already inconsistent with the
error from OFFSET -1 ROWS.)

-- 
Andrew (irc:RhodiumToad)

diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index b5d3d3a071..330adb8c37 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -1399,10 +1399,11 @@ OFFSET <replaceable class="parameter">start</replaceable>
 OFFSET <replaceable class="parameter">start</replaceable> { ROW | ROWS }
 FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } ONLY
 </synopsis>
-    In this syntax, to write anything except a simple integer constant for
+    In this syntax, to write anything except a simple integer constant,
+    parameter, or variable name for
     <replaceable class="parameter">start</replaceable> or <replaceable
-    class="parameter">count</replaceable>, you must write parentheses
-    around it.
+    class="parameter">count</replaceable>, you should write parentheses
+    around it (this is a <productname>PostgreSQL</productname> extension).
     If <replaceable class="parameter">count</replaceable> is
     omitted in a <literal>FETCH</literal> clause, it defaults to 1.
     <literal>ROW</literal>
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index babb62dae1..e441c555a4 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -451,7 +451,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 
 %type <node>    fetch_args limit_clause select_limit_value
                 offset_clause select_offset_value
-                select_offset_value2 opt_select_fetch_first_value
+                select_fetch_first_value
 %type <ival>    row_or_rows first_or_next
 
 %type <list>    OptSeqOptList SeqOptList OptParenthesizedSeqOptList
@@ -11570,15 +11570,23 @@ limit_clause:
                              parser_errposition(@1)));
                 }
             /* SQL:2008 syntax */
-            | FETCH first_or_next opt_select_fetch_first_value row_or_rows ONLY
+            /* to avoid shift/reduce conflicts, handle the optional value with
+             * a separate production rather than an opt_ expression.  The fact
+             * that ONLY is fully reserved means that this way, we defer any
+             * decision about what rule reduces ROW or ROWS to the point where
+             * we can see the ONLY token in the lookahead slot.
+             */
+            | FETCH first_or_next select_fetch_first_value row_or_rows ONLY
                 { $$ = $3; }
+            | FETCH first_or_next row_or_rows ONLY
+                { $$ = makeIntConst(1, -1); }
         ;
 
 offset_clause:
             OFFSET select_offset_value
                 { $$ = $2; }
             /* SQL:2008 syntax */
-            | OFFSET select_offset_value2 row_or_rows
+            | OFFSET select_fetch_first_value row_or_rows
                 { $$ = $2; }
         ;
 
@@ -11597,21 +11605,16 @@ select_offset_value:
 
 /*
  * Allowing full expressions without parentheses causes various parsing
- * problems with the trailing ROW/ROWS key words.  SQL only calls for
- * constants, so we allow the rest only with parentheses.  If omitted,
- * default to 1.
- */
-opt_select_fetch_first_value:
-            SignedIconst                        { $$ = makeIntConst($1, @1); }
-            | '(' a_expr ')'                    { $$ = $2; }
-            | /*EMPTY*/                            { $$ = makeIntConst(1, -1); }
-        ;
-
-/*
- * Again, the trailing ROW/ROWS in this case prevent the full expression
- * syntax.  c_expr is the best we can do.
+ * problems with the trailing ROW/ROWS key words.  SQL spec only calls for
+ * <simple value specification>, which is either a literal or a parameter (but
+ * an <SQL parameter reference> could be an identifier, bringing up conflicts
+ * with ROW/ROWS). We solve this by leveraging the presense of ONLY (see above)
+ * to determine whether the expression is missing rather than trying to make it
+ * optional in this rule.
+ *
+ * c_expr covers all the spec-required cases (and more).
  */
-select_offset_value2:
+select_fetch_first_value:
             c_expr                                    { $$ = $1; }
         ;