Subject: Using COPY command in pqlib

From: Lukáš Sobotka <[email-masked]>
Date: Thu, 18 Jul 2019 15:57:55 +0200

Hi guys,

I would like to ask you on correct way how to use pqlib for copying data.
We tried to do it according documentation but some problems occurred.

We have a multi-threaded application, all threads hold own non-blocking
connection into the remote database. Each thread is collecting data and
when a buffer is filled they copy data to remote DB as quick as possible.
Copying can run several times per minute. The postgresql related part of
thread main loop is briefly shown in attached file main.c.

Most of the time copying works well but big delay sometimes occurred. I
captured pcap (see https://ibb.co/2MhmSTZ) and the delay is between getting
PGRES_COPY_IN from PQresultStatus function and pushing data. But I am sure
that commands PQputCopyData and PQputCopyEnd are called immediately after
client gets PQexec.

What am I missing? Is there some other command which we should use?

We are using PostgreSQL 9.6 (PostgreSQL 9.6.10 on x86_64-pc-linux-gnu,
compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28), 64-bit).

Best regards,
Lukas

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From: Tom Lane <[email-masked]>
Date: Thu, 18 Jul 2019 18:26:41 -0400

> What am I missing? Is there some other command which we should use?

Well, you didn't show us any of the relevant parts of your code.
There is extra stuff you need to deal with if you're running the
connection in nonblock mode.

--- what are you doing when PQputCopyData returns zero?

The documentation says you should wait for write-ready and try again.
(You could just try again immediately, I suppose, but that would make it
a busy-wait loop which doesn't seem like a great idea.)

--- what are you doing after PQputCopyEnd?

The documentation says

    The result is 1 if the termination message was sent; or in nonblocking
    mode, this may only indicate that the termination message was
    successfully queued. (In nonblocking mode, to be certain that the data
    has been sent, you should next wait for write-ready and call PQflush,
    repeating until it returns zero.) Zero indicates that the function
    could not queue the termination message because of full buffers; this
    will only happen in nonblocking mode. (In this case, wait for
    write-ready and try the PQputCopyEnd call again.) If a hard error
    occurs, -1 is returned; you can use PQerrorMessage to retrieve
    details.

			regards, tom lane

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From: Lukáš Sobotka <[email-masked]>
Date: Fri, 19 Jul 2019 12:34:19 +0200

Hi Tom,

I see we are not using copy in correct way.

Many thanks for your time.
Lukas