Identity columns

This is the SQL standard-conforming variant of PostgreSQL's serial
columns.  It fixes a few usability issues that serial columns have:

- CREATE TABLE / LIKE copies default but refers to same sequence
- cannot add/drop serialness with ALTER TABLE
- dropping default does not drop sequence
- need to grant separate privileges to sequence
- other slight weirdnesses because serial is some kind of special macro

Reviewed-by: Vitaly Burovoy <vitaly.burovoy@gmail.com>

src/backend/executor/execExpr.c

@@ -1993,6 +1993,18 @@ ExecInitExprRec(Expr *node, PlanState *parent, ExprState *state,
 				break;
 			}
 
+		case T_NextValueExpr:
+			{
+				NextValueExpr *nve = (NextValueExpr *) node;
+
+				scratch.opcode = EEOP_NEXTVALUEEXPR;
+				scratch.d.nextvalueexpr.seqid = nve->seqid;
+				scratch.d.nextvalueexpr.seqtypid = nve->typeId;
+
+				ExprEvalPushStep(state, &scratch);
+				break;
+			}
+
 		default:
 			elog(ERROR, "unrecognized node type: %d",
 				 (int) nodeTag(node));