Function transformJoinUsingClause

src/backend/parser/parse_clause.c:448–501

transformJoinUsingClause() * Build a complete ON clause from a partially-transformed USING list. * We are given lists of nodes representing left and right match columns. * Result is a transformed qualification expression. */

 *	  Result is a transformed qualification expression.
 */
static Node *
transformJoinUsingClause(ParseState *pstate,
						 List *leftVars, List *rightVars)
{
	Node	   *result;
	List	   *andargs = NIL;
	ListCell   *lvars,
			   *rvars;

	/*
	 * We cheat a little bit here by building an untransformed operator tree
	 * whose leaves are the already-transformed Vars.  This requires collusion
	 * from transformExpr(), which normally could be expected to complain
	 * about already-transformed subnodes.  However, this does mean that we
	 * have to mark the columns as requiring SELECT privilege for ourselves;
	 * transformExpr() won't do it.
	 */
	forboth(lvars, leftVars, rvars, rightVars)
	{
		Var		   *lvar = (Var *) lfirst(lvars);
		Var		   *rvar = (Var *) lfirst(rvars);
		A_Expr	   *e;

		/* Require read access to the join variables */
		markVarForSelectPriv(pstate, lvar);
		markVarForSelectPriv(pstate, rvar);

		/* Now create the lvar = rvar join condition */
		e = makeSimpleA_Expr(AEXPR_OP, "=",
							 (Node *) copyObject(lvar), (Node *) copyObject(rvar),
							 -1);

		/* Prepare to combine into an AND clause, if multiple join columns */
		andargs = lappend(andargs, e);
	}

	/* Only need an AND if there's more than one join column */
	if (list_length(andargs) == 1)
		result = (Node *) linitial(andargs);
	else
		result = (Node *) makeBoolExpr(AND_EXPR, andargs, -1);

	/*
	 * Since the references are already Vars, and are certainly from the input
	 * relations, we don't have to go through the same pushups that
	 * transformJoinOnClause() does.  Just invoke transformExpr() to fix up
	 * the operators, and we're done.
	 */
	result = transformExpr(pstate, result, EXPR_KIND_JOIN_USING);

	result = coerce_to_boolean(pstate, result, "JOIN/USING");

	return result;
}

/* transformJoinOnClause()
 *	  Transform the qual conditions for JOIN/ON.