EXCEPT over no columns reports ERROR: unexpected gang size

[z-wenlin]

Bug Report

postgres=# create table tt2(id int);
postgres=# insert into tt2 select generate_series(1,5);
postgres=# set optimizer=off;
SET
postgres=# explain select from generate_series(1,10) except select from tt2;
ERROR:  unexpected gang size: 3 (nodeMotion.c:732)

This only happens when none of the columns are hashable.

postgres=# explain  select *  from generate_series(1,10) except select *  from tt2;
                                             QUERY PLAN
----------------------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..917.51 rows=30 width=8)
   ->  HashSetOp Except  (cost=0.00..917.11 rows=10 width=8)
         ->  Append  (cost=0.00..836.85 rows=32103 width=8)
               ->  Redistribute Motion 1:3  (slice2; segments: 1)  (cost=0.00..0.34 rows=3 width=8)
                     Hash Key: "*SELECT* 1".generate_series
                     ->  Subquery Scan on "*SELECT* 1"  (cost=0.00..0.20 rows=10 width=8)
                           ->  Function Scan on generate_series  (cost=0.00..0.10 rows=10 width=4)
               ->  Subquery Scan on "*SELECT* 2"  (cost=0.00..676.00 rows=32100 width=8)
                     ->  Seq Scan on tt2  (cost=0.00..355.00 rows=32100 width=4)
 Optimizer: Postgres-based planner
(10 rows)

Greenplum version or build

Greenplum Database 7.1.0-addcol+dev.15.gb8411c0 build dev

OS version and uname -a

RHEL7

autoconf options used ( config.status --config )

Installation information ( pg_config )

Expected behavior

Actual behavior

Step to reproduce the behavior