Fix duplicate distribution keys from subqueries.

The parser normally prevents duplicate distribution keys in main query
grammar, but they can still originate from subqueries - particularly in
window function PARTITION BY clauses with equivalent expressions.

When duplicate distribution keys are detected, we now properly fall back
to alternative processing. The caller already handles this fallback
scenario correctly; this fix ensures the fallback is triggered when
needed.

Author: avamingli

contrib/pax_storage/src/test/regress/expected/create_table_distpol.out

@@ -128,9 +128,7 @@ select distkey from gp_distribution_policy where localoid = 'distpol_person_copy
 RESET gp_create_table_random_default_distribution;
 -- Test duplicate distribute keys
 CREATE TABLE ctas_dup_dk as SELECT distinct age as c1, age as c2 from distpol_person; 
-ERROR:  duplicate DISTRIBUTED BY column 'c1'
 SELECT distinct age c1, age c2 into ctas_dup_dk_1 from distpol_person;
-ERROR:  duplicate DISTRIBUTED BY column 'c1'
 --
 -- Test deriving distribution key from the query's distribution in
 -- CREATE TABLE AS

src/backend/cdb/cdbllize.c

@@ -161,6 +161,7 @@ get_partitioned_policy_from_path(PlannerInfo *root, Path *path)
 	ListCell   *dk_cell;
 	ListCell   *ec_cell;
 	ListCell   *em_cell;
+	bool duplicate_keys_detected = false;
 
 	/*
 	 * Is it a Hashed distribution?
@@ -184,6 +185,9 @@ get_partitioned_policy_from_path(PlannerInfo *root, Path *path)
 		DistributionKey *dk = lfirst(dk_cell);
 		bool		found_expr = false;
 
+		if (duplicate_keys_detected)
+			return NULL;
+
 		foreach(ec_cell, dk->dk_eclasses)
 		{
 			EquivalenceClass *ec = lfirst(ec_cell);
@@ -230,10 +234,17 @@ get_partitioned_policy_from_path(PlannerInfo *root, Path *path)
 						Assert(list_length(policykeys) < MaxPolicyAttributeNumber);
 
 						if (list_member_int(policykeys, attno))
-							ereport(ERROR,
-									(errcode(ERRCODE_DUPLICATE_COLUMN),
-									 errmsg("duplicate DISTRIBUTED BY column '%s'",
-											target->resname ? target->resname : "???")));
+						{
+							/*
+							 * Although the parser prevents duplicate distribution keys in regular grammar,
+							 * they can still occur in subqueries(e.g., window function PARTITION BY columns
+							 * with equivalent expressions).
+							 * We fall back to an alternative approach when duplicate distribution keys are detected;
+							 * the caller handles this scenario appropriately.
+							 */
+							duplicate_keys_detected = true;
+							break;
+						}
 
 						/*
 						 * We know the btree operator family corresponding to

src/test/regress/expected/create_table_distpol.out

@@ -128,9 +128,7 @@ select distkey from gp_distribution_policy where localoid = 'distpol_person_copy
 RESET gp_create_table_random_default_distribution;
 -- Test duplicate distribute keys
 CREATE TABLE ctas_dup_dk as SELECT distinct age as c1, age as c2 from distpol_person; 
-ERROR:  duplicate DISTRIBUTED BY column 'c1'
 SELECT distinct age c1, age c2 into ctas_dup_dk_1 from distpol_person;
-ERROR:  duplicate DISTRIBUTED BY column 'c1'
 --
 -- Test deriving distribution key from the query's distribution in
 -- CREATE TABLE AS

src/test/regress/expected/gp_create_table.out

@@ -202,3 +202,8 @@ c1 int,c2 int,c3 int,c4 int,c5 int,c6 int,c7 int,c8 int,c9 int,c10 int,c11 int,c
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 );
 ERROR:  tables can have at most 1600 columns
+create table dup_key_t(a int, b int, c int) distributed randomly;
+create temp table ctas_dump_key_t as select rn, a, b from
+ (select a, b, row_number() over (partition by a, b order by c) rn from dup_key_t where dup_key_t.a = dup_key_t.b) x;
+NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column(s) named 'rn' as the Apache Cloudberry data distribution key for this table.
+HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.