Re: Parallel Seq Scan

On 2015/11/12 4:26, Robert Haas wrote:
> On Wed, Nov 11, 2015 at 12:59 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>> I have a first query
>>
>> I looked on EXPLAIN ANALYZE output and the numbers of filtered rows are
>> differen
>
> Hmm, I see I was right about people finding more bugs once this was
> committed. That didn't take long.

I encountered one more odd behavior:

postgres=# EXPLAIN ANALYZE SELECT abalance FROM pgbench_accounts WHERE aid
= 23466;
QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------
Gather (cost=1000.00..65207.88 rows=1 width=4) (actual
time=17450.595..17451.151 rows=1 loops=1)
Number of Workers: 4
-> Parallel Seq Scan on pgbench_accounts (cost=0.00..64207.78 rows=1
width=4) (actual time=55.934..157001.134 rows=2 loops=1)
Filter: (aid = 23466)
Rows Removed by Filter: 18047484
Planning time: 0.198 ms
Execution time: 17453.565 ms
(7 rows)

The #rows removed here is almost twice the number of rows in the table
(10m). Also, the #rows selected shown is 2 for Parallel Seq Scan whereas
only 1 row is selected.

Thanks,
Amit