Revert rebase merge of #7620 (#7626)

Because we want to track PR numbers and to make backporting easy we
(pretty much always) use squash-merges when merging to master. We
accidentally used a rebase merge for PR #7620. This reverts those
changes so we can redo the merge using squash merge.

This reverts all commits from eedb607 to 9e71750.

src/backend/distributed/planner/merge_planner.c

@@ -182,6 +182,14 @@ CreateRouterMergePlan(Oid targetRelationId, Query *originalQuery, Query *query,
 		return distributedPlan;
 	}
 
+	Var *insertVar =
+		FetchAndValidateInsertVarIfExists(targetRelationId, originalQuery);
+	if (insertVar &&
+		!IsDistributionColumnInMergeSource((Expr *) insertVar, originalQuery, true))
+	{
+		ereport(ERROR, (errmsg("MERGE INSERT must use the source table "
+							   "distribution column value")));
+	}
 
 	Job *job = RouterJob(originalQuery, plannerRestrictionContext,
 						 &distributedPlan->planningError);
@@ -1116,27 +1124,6 @@ DeferErrorIfRoutableMergeNotSupported(Query *query, List *rangeTableList,
 								"repartitioning")));
 		return deferredError;
 	}
-
-
-	/*
-	 * If execution has reached this point, it indicates that the query can be delegated to the worker.
-	 * However, before proceeding with this delegation, we need to confirm that the user is utilizing
-	 * the distribution column of the source table in the Insert variable.
-	 * If this is not the case, we should refrain from pushing down the query.
-	 * This is just a deffered error which will be handle by caller.
-	 */
-
-	Var *insertVar =
-		FetchAndValidateInsertVarIfExists(targetRelationId, query);
-	if (insertVar &&
-		!IsDistributionColumnInMergeSource((Expr *) insertVar, query, true))
-	{
-		ereport(DEBUG1, (errmsg(
-							 "MERGE INSERT must use the source table distribution column value for push down to workers. Otherwise, repartitioning will be applied")));
-		return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
-							 "MERGE INSERT must use the source table distribution column value for push down to workers. Otherwise, repartitioning will be applied",
-							 NULL, NULL);
-	}
 	return NULL;
 }