Function transformCollateClause

src/backend/parser/parse_expr.c:2828–2854

* Handle an explicit COLLATE clause. * * Transform the argument, and look up the collation name. */

 * Transform the argument, and look up the collation name.
 */
static Node *
transformCollateClause(ParseState *pstate, CollateClause *c)
{
	CollateExpr *newc;
	Oid			argtype;

	newc = makeNode(CollateExpr);
	newc->arg = (Expr *) transformExprRecurse(pstate, c->arg);

	argtype = exprType((Node *) newc->arg);

	/*
	 * The unknown type is not collatable, but coerce_type() takes care of it
	 * separately, so we'll let it go here.
	 */
	if (!type_is_collatable(argtype) && argtype != UNKNOWNOID)
		ereport(ERROR,
				(errcode(ERRCODE_DATATYPE_MISMATCH),
				 errmsg("collations are not supported by type %s",
						format_type_be(argtype)),
				 parser_errposition(pstate, c->location)));

	newc->collOid = LookupCollation(pstate, c->collname, c->location);
	newc->location = c->location;

	return (Node *) newc;
}

/*
 * Transform a "row compare-op row" construct