INSERT INTO SELECT, Why Parallelism is not selected?

I have just notice that the parallelism is off even for the select
part of the query mentioned in the $subject.  I see the only reason it
is not getting parallel because we block the parallelism if the query
type is not SELECT.  I don't see any reason for not selecting the
parallelism for this query.  I have quickly hacked the code to enable
the parallelism for this query.  I can see there is a significant
improvement if we can enable the parallelism in this case.  For an
experiment, I have just relaxed a couple of checks, maybe if we think
that it's good to enable the parallelism for this case we can try to
put better checks which are specific for this quey.

No parallel:
postgres[36635]=# explain analyze insert into t2 select * from t where a < 100;
 Insert on t2  (cost=0.00..29742.00 rows=100 width=105) (actual
time=278.300..278.300 rows=0 loops=1)
   ->  Seq Scan on t  (cost=0.00..29742.00 rows=100 width=105) (actual
time=0.061..277.330 rows=99 loops=1)
         Filter: (a < 100)
         Rows Removed by Filter: 999901
 Planning Time: 0.093 ms
 Execution Time: 278.330 ms
(6 rows)

With parallel
 Insert on t2  (cost=1000.00..23460.33 rows=100 width=105) (actual
time=108.410..108.410 rows=0 loops=1)
   ->  Gather  (cost=1000.00..23460.33 rows=100 width=105) (actual
time=0.306..108.973 rows=99 loops=1)
         Workers Planned: 2
         Workers Launched: 2
         ->  Parallel Seq Scan on t  (cost=0.00..22450.33 rows=42
width=105) (actual time=66.396..101.979 rows=33 loops=3)
               Filter: (a < 100)
               Rows Removed by Filter: 333300
 Planning Time: 0.154 ms
 Execution Time: 110.158 ms
(9 rows)

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com