Re: Asynchronous Append on postgres_fdw nodes.

On 6/4/20 11:00 AM, Kyotaro Horiguchi wrote:
> Removed a useless variable PgFdwScanState.result_ready.
> Removed duplicate code from remove_async_node() by using move_to_next_waiter().
> Done some minor cleanups.
> 
I am reviewing your code.
A couple of variables are no longer needed (see changes.patch in attachment.

Something about the cost of an asynchronous plan:

At the simple query plan (see below) I see:
1. Startup cost of local SeqScan is equal 0, ForeignScan - 100. But 
startup cost of Append is 0.
2. Total cost of an Append node is a sum of the subplans. Maybe in the 
case of asynchronous append we need to use some reduce factor?

explain select * from parts;

With Async Append:
=====================

  Append  (cost=0.00..2510.30 rows=106780 width=8)
    Async subplans: 3
    ->  Async Foreign Scan on part_1 parts_2  (cost=100.00..177.80 
rows=2260 width=8)
    ->  Async Foreign Scan on part_2 parts_3  (cost=100.00..177.80 
rows=2260 width=8)
    ->  Async Foreign Scan on part_3 parts_4  (cost=100.00..177.80 
rows=2260 width=8)
    ->  Seq Scan on part_0 parts_1  (cost=0.00..1443.00 rows=100000 width=8)

Without Async Append:
=====================

  Append  (cost=0.00..2510.30 rows=106780 width=8)
    ->  Seq Scan on part_0 parts_1  (cost=0.00..1443.00 rows=100000 width=8)
    ->  Foreign Scan on part_1 parts_2  (cost=100.00..177.80 rows=2260 
width=8)
    ->  Foreign Scan on part_2 parts_3  (cost=100.00..177.80 rows=2260 
width=8)
    ->  Foreign Scan on part_3 parts_4  (cost=100.00..177.80 rows=2260 
width=8)

-- 
Andrey Lepikhov
Postgres Professional
https://postgrespro.com
The Russian Postgres Company