corresponding.part.c

else if(linitial(stmt->correspondingClause) == NULL)
{
	// CORRESPONDING clause, find matching column names from both tables. If there are none then it is a syntax error.

	Query	*largQuery;
	Query	*rargQuery;
	List	*matchingColumns;

	/* Analyze left query to resolve column names. */
	largQuery = parse_sub_analyze((Node *) stmt->larg, pstate, NULL, false);

	/* Analyze right query to resolve column names. */
	rargQuery = parse_sub_analyze((Node *) stmt->rarg, pstate, NULL, false);

	/* Find matching columns from both queries. */
	matchingColumns = determineMatchingColumns(largQuery->targetList,
											   rargQuery->targetList);

	op->correspondingColumns = matchingColumns;
	op->hasCorrespondingBy = false;

	/* If matchingColumns is empty, there is an error. At least one column in the select lists must have the same name. */
	if(list_length(matchingColumns) == 0)
	{
		ereport(ERROR, (errcode(ERRCODE_SYNTAX_ERROR),
						errmsg("%s queries with a CORRESPONDING clause must have at least one column with the same name",
						context)));
	}

	// Create subquery for larg, selecting column names from matchingColumns.
	stmt->larg = createSubqueryForCorresponding(matchingColumns, stmt->larg);

	// Assign newly generated query to original left query.
	op->larg = transformSetOperationTree(pstate, stmt->larg,
										 false,
										 <argetlist);

	// Create subquery for rarg, selecting column names from matchingColumns.
	stmt->rarg = createSubqueryForCorresponding(matchingColumns, stmt->rarg);

	// Assign newly generated query to original right query.
	op->rarg = transformSetOperationTree(pstate, stmt->rarg,
										 false,
										 &rtargetlist);
}