Handle CTE aliases accurately

Basically, make sure to update the column name with the CTEs alias
if we need to do so.

src/backend/distributed/planner/cte_inline.c

@@ -46,6 +46,8 @@ typedef struct inline_cte_walker_context
 	int levelsup;
 	int refcount;              /* number of remaining references */
 	Query *ctequery;           /* query to substitute */
+
+	List *aliascolnames;  /* citus addition to Postgres' inline_cte_walker_context */
 } inline_cte_walker_context;
 
 /* copy & paste from Postgres source, moved into a function for readability */
@@ -265,6 +267,7 @@ inline_cte(Query *mainQuery, CommonTableExpr *cte)
 	context.levelsup = -1;
 	context.refcount = cte->cterefcount;
 	context.ctequery = castNode(Query, cte->ctequery);
+	context.aliascolnames = cte->aliascolnames;
 
 	(void) inline_cte_walker((Node *) mainQuery, &context);
 
@@ -333,6 +336,28 @@ inline_cte_walker(Node *node, inline_cte_walker_context *context)
 			rte->subquery = newquery;
 			rte->security_barrier = false;
 
+			List *columnAliasList = context->aliascolnames;
+			int columnAliasCount = list_length(columnAliasList);
+			int columnIndex = 1;
+			for (; columnIndex < list_length(rte->subquery->targetList) + 1; ++columnIndex)
+			{
+				/*
+				 * Rename the column only if a column alias is defined.
+				 * Notice that column alias count could be less than actual
+				 * column count. We only use provided aliases and keep the
+				 * original column names if no alias is defined.
+				 */
+				if (columnAliasCount >= columnIndex)
+				{
+					Value *columnAlias = (Value *) list_nth(columnAliasList, columnIndex - 1);
+					Assert(IsA(columnAlias, String));
+					TargetEntry *targetEntry =
+						list_nth(rte->subquery->targetList, columnIndex - 1);
+					Assert(IsA(columnAlias, String));
+					targetEntry->resname = strVal(columnAlias);
+				}
+			}
+
 			/* Zero out CTE-specific fields */
 			rte->ctename = NULL;
 			rte->ctelevelsup = 0;