Deveel.Data.Sql.Query.QueryTablePlanner.JoinToSingle C# (CSharp) Method

        private TablePlan JoinToSingle(IList<TablePlan> allPlans)
        {
            // If there are no plans then return null
            if (allPlans.Count == 0)
                return null;

            if (allPlans.Count == 1)
                // Return early if there is only 1 table.
                return allPlans[0];

            // Make a working copy of the plan list.
            var workingPlanList = new List<TablePlan>(allPlans);

            // We go through each plan in turn.
            while (workingPlanList.Count > 1) {
                var leftPlan = workingPlanList[0];
                var rightPlan = workingPlanList[1];

                // First we need to determine if the left and right plan can be
                // naturally joined.
                int status = AssertBeNaturalJoin(leftPlan, rightPlan);
                if (status == 0) {
                    // Yes they can so join them
                    var newPlan = NaturallyJoinPlans(leftPlan, rightPlan);

                    // Remove the left and right plan from the list and add the new plan
                    workingPlanList.Remove(leftPlan);
                    workingPlanList.Remove(rightPlan);
                    workingPlanList.Insert(0, newPlan);
                } else if (status == 1) {
                    // No we can't because of a right join clash, so we join the left
                    // plan right in hopes of resolving the clash.
                    var newPlan = NaturallyJoinPlans(leftPlan, leftPlan.RightPlan);
                    workingPlanList.Remove(leftPlan);
                    workingPlanList.Remove(leftPlan.RightPlan);
                    workingPlanList.Insert(0, newPlan);
                } else if (status == 2) {
                    // No we can't because of a left join clash, so we join the left
                    // plan left in hopes of resolving the clash.
                    var newPlan = NaturallyJoinPlans(leftPlan, leftPlan.LeftPlan);
                    workingPlanList.Remove(leftPlan);
                    workingPlanList.Remove(leftPlan.LeftPlan);
                    workingPlanList.Insert(0, newPlan);
                } else {
                    throw new InvalidOperationException(String.Format("Natural join assessed status {0} is unknown.", status));
                }
            }

            // Return the working plan of the merged tables.
            return workingPlanList[0];
        }