Fix SQL:2008 FETCH FIRST syntax to allow parameters.

OFFSET <x> ROWS FETCH FIRST <y> ROWS ONLY syntax is supposed to accept
<simple value specification>, which includes parameters as well as
literals. When this syntax was added all those years ago, it was done
inconsistently, with <x> and <y> being different subsets of the
standard syntax.

Rectify that by making <x> and <y> accept the same thing, and allowing
either a (signed) numeric literal or a c_expr there, which allows for
parameters, variables, and parenthesized arbitrary expressions.

Per bug #15200 from Lukas Eder.

Backpatch all the way, since this has been broken from the start.

Discussion: https://postgr.es/m/877enz476l.fsf@news-spur.riddles.org.uk
Discussion: http://postgr.es/m/152647780335.27204.16895288237122418685@wrigleys.postgresql.org

doc/src/sgml/ref/select.sgml

@@ -1399,10 +1399,12 @@ 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, the <replaceable class="parameter">start</replaceable>
-    <replaceable class="parameter">start</replaceable> or <replaceable
+    or <replaceable class="parameter">count</replaceable> value is required by
-    class="parameter">count</replaceable>, you must write parentheses
+    the standard to be a literal constant, a parameter, or a variable name;
-    around it.
+    as a <productname>PostgreSQL</productname> extension, other expressions
+    are allowed, but will generally need to be enclosed in parentheses to avoid
+    ambiguity.
     If <replaceable class="parameter">count</replaceable> is
     omitted in a <literal>FETCH</literal> clause, it defaults to 1.
     <literal>ROW</literal>

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 I_or_F_const
 %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; }
 		;
 
@@ -11597,22 +11605,31 @@ select_offset_value:
 
 /*
  * Allowing full expressions without parentheses causes various parsing
- * problems with the trailing ROW/ROWS key words.  SQL only calls for
+ * problems with the trailing ROW/ROWS key words.  SQL spec only calls for
- * constants, so we allow the rest only with parentheses.  If omitted,
+ * <simple value specification>, which is either a literal or a parameter (but
- * default to 1.
+ * an <SQL parameter reference> could be an identifier, bringing up conflicts
+ * with ROW/ROWS). We solve this by leveraging the presence of ONLY (see above)
+ * to determine whether the expression is missing rather than trying to make it
+ * optional in this rule.
+ *
+ * c_expr covers almost all the spec-required cases (and more), but it doesn't
+ * cover signed numeric literals, which are allowed by the spec. So we include
+ * those here explicitly. We need FCONST as well as ICONST because values that
+ * don't fit in the platform's "long", but do fit in bigint, should still be
+ * accepted here. (This is possible in 64-bit Windows as well as all 32-bit
+ * builds.)
  */
-opt_select_fetch_first_value:
+select_fetch_first_value:
-			SignedIconst						{ $$ = makeIntConst($1, @1); }
+			c_expr									{ $$ = $1; }
-			| '(' a_expr ')'					{ $$ = $2; }
+			| '+' I_or_F_const
-			| /*EMPTY*/							{ $$ = makeIntConst(1, -1); }
+				{ $$ = (Node *) makeSimpleA_Expr(AEXPR_OP, "+", NULL, $2, @1); }
+			| '-' I_or_F_const
+				{ $$ = doNegate($2, @1); }
 		;
 
-/*
+I_or_F_const:
- * Again, the trailing ROW/ROWS in this case prevent the full expression
+			Iconst									{ $$ = makeIntConst($1,@1); }
- * syntax.  c_expr is the best we can do.
+			| FCONST								{ $$ = makeFloatConst($1,@1); }
- */
-select_offset_value2:
-			c_expr									{ $$ = $1; }
 		;
 
 /* noise words */