Fixes greenplum db-driver's assign_sequential_id

when run with many segments that showed PL/Python error by forcing
nested loop join algorithm

database/db-driver/greenplum/db-assign_sequential_id

@@ -67,12 +67,17 @@ CREATE TEMP TABLE idstats_bases AS
   DISTRIBUTED RANDOMLY;
 
 CREATE TEMP TABLE idstats_distributor as
-  SELECT generate_series(1, 1000000) id
+  SELECT generate_series(1, 100000) id
   DISTRIBUTED BY (id);
 
 ANALYZE idstats_bases;
 ANALYZE idstats_distributor;
 
+-- Have GP lay down data in a safe way for seqassign()
+SET enable_hashjoin=off;
+SET enable_mergejoin=off;
+SET enable_nestloop=off;
+
 SELECT seqassign(gpsid, true, pids, offsets)
   FROM idstats_bases a, idstats_distributor b
   WHERE a.gpsid = b.gp_segment_id;