Horton

Re: Asynchronous Append on postgres_fdw nodes.

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

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From: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: etsuro.fujita@gmail.com, movead.li@highgo.ca, pgsql-hackers@lists.postgresql.org
Date: 2020-06-09T09:20:42Z
Lists: pgsql-hackers

Attachments

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

Commits

  1. Improve comments for trivial_subqueryscan().

  2. Disable asynchronous execution if using gating Result nodes.

  3. Allow asynchronous execution in more cases.

  4. Doc: Further update documentation for asynchronous execution.

  5. Fix rescanning of async-aware Append nodes.

  6. Doc: Update documentation for asynchronous execution.

  7. Fix EXPLAIN ANALYZE for async-capable nodes.

  8. Minor code cleanup in asynchronous execution support.

  9. Adjust input value to WaitEventSetWait() in ExecAppendAsyncEventWait().

  10. Add support for asynchronous execution.