Обсуждение: weird interaction between asynchronous queries and pg_sleep

Consider the following snippet

create table data as select generate_series(1,1000000) s;

do $d$
begin
  PERFORM * FROM dblink_connect('test','');

  PERFORM * from dblink_send_query('test', 'SELECT * FROM data');

  LOOP
    if dblink_is_busy('test') = 0
    THEN
      PERFORM * FROM dblink_get_result('test') AS R(V int);
      PERFORM * FROM dblink_get_result('test') AS R(V int);
      RETURN;
    END IF;

    PERFORM pg_sleep(.001);
  END LOOP;

  PERFORM * FROM dblink_disconnect('test');
END;
$d$;

What's interesting here is that, when I vary the sleep parameter, I get:
0: .4 seconds (per top, this is busywait), same as running synchronous.
0.000001: 1.4 seconds
0.001: 2.4 seconds
0.01: 10.6 seconds
0.1: does not terminate

This effect is only noticeable when the remote query is returning
volumes of data.  My question is, is there any way to sleep loop
client side without giving up 3x performance penalty?  Why is that
that when more local sleep queries are executed, performance improves?

merlin

On Thu, Apr 8, 2021 at 1:05 PM Merlin Moncure <mmoncure@gmail.com> wrote:
> This effect is only noticeable when the remote query is returning
> volumes of data.  My question is, is there any way to sleep loop
> client side without giving up 3x performance penalty?  Why is that
> that when more local sleep queries are executed, performance improves?


Looking at this more, it looks like that when sleeping with pg_sleep,
libpq does not receive the data.  I think for this type of pattern to
work correctly, dblink would need a custom sleep function wrapping
poll (or epoll) that consumes input on the socket when signalled read
ready.

merlin