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Thread: Copy command hangs in poll() function

Copy command hangs in poll() function

From
Akshay Joshi
Date:
Hello 

As per documentation http://initd.org/psycopg/docs/advanced.html Copy commands are not supported in asynchronous mode, but this will be probably implemented in a future release. In pgAdmin4 (open source) we have used async connection to run the SQL query on PostgreSQL database and uses poll() function to poll the results. When user runs any COPY command application hangs at poll() function.

If copy command not supported then at least it should not hang the application. poll() function should return the proper error message. 

Can you please provide that fix until support for copy command won't come.   

--
Akshay Joshi
Sr. Software Architect


Phone: +91 20-3058-9517
Mobile: +91 976-788-8246

Re: Copy command hangs in poll() function

From
Daniele Varrazzo
Date:
On Thu, Oct 4, 2018 at 6:35 AM Akshay Joshi <akshay.joshi@enterprisedb.com> wrote:
Hello 

As per documentation http://initd.org/psycopg/docs/advanced.html Copy commands are not supported in asynchronous mode, but this will be probably implemented in a future release. In pgAdmin4 (open source) we have used async connection to run the SQL query on PostgreSQL database and uses poll() function to poll the results. When user runs any COPY command application hangs at poll() function. 

If copy command not supported then at least it should not hang the application. poll() function should return the proper error message. 

Can you please provide that fix until support for copy command won't come.
 
Uhm, I see:

    In [2]: cnn = psycopg2.connect("")
    In [3]: cur = cnn.cursor()
    In [4]: cur.execute("copy (select 1 as x) to stdout")
    ---------------------------------------------------------------------------
    ProgrammingError                          Traceback (most recent call last)
    <ipython-input-4-62e3af3d121e> in <module>()
    ----> 1 cur.execute("copy (select 1 as x) to stdout")
    ProgrammingError: can't execute COPY TO: use the copy_to() method instead

but:

    In [6]: cnn = psycopg2.connect("", async=True)
    In [7]: cnn.poll()
    Out[7]: 2
    In [8]: cnn.poll()
    Out[8]: 1
    In [9]: cnn.poll()
    Out[9]: 0

    In [12]: cur.execute("copy (select 1 as x) to stdout")
    In [14]: cnn.poll()
    ^C -- hung

So yes, I'll take a look if there is a way to detect we get in this state. But I'm not sure it's possible, because we might not have received a result yet that tells we are in a state not allowed (in a COPY).

Observation: maybe EnterpriseDB has resources enough to help us debugging and fixing this issue, or maybe even implement async copy?

-- Daniele

Re: Copy command hangs in poll() function

From
Daniele Varrazzo
Date:
On Thu, Oct 4, 2018 at 11:26 AM Daniele Varrazzo
<daniele.varrazzo@gmail.com> wrote:

> Uhm, I see:

Issue https://github.com/psycopg/psycopg2/issues/781 opened.

-- Daniele

Re: Copy command hangs in poll() function

From
Federico Di Gregorio
Date:
On 10/04/2018 12:26 PM, Daniele Varrazzo wrote:
> On Thu, Oct 4, 2018 at 6:35 AM Akshay Joshi 
> <akshay.joshi@enterprisedb.com <mailto:akshay.joshi@enterprisedb.com>> 
> wrote:
> 
>     Hello
> 
>     As per documentation http://initd.org/psycopg/docs/advanced.html
>     <http://initd.org/psycopg/docs/advanced.html> Copy commands are not
>     supported in asynchronous mode, but this will be probably
>     implemented in a future release. In pgAdmin4 (open source) we have
>     used async connection to run the SQL query on PostgreSQL database
>     and uses poll() function to poll the results. When user runs any
>     *COPY *command application hangs at poll() function. 
> 
> 
>     If copy command not supported then at least it should not hang the
>     application. poll() function should return the proper error message.
> 
>     Can you please provide that fix until support for copy command won't
>     come.
> 
> 
> Uhm, I see:
> 
>      In [2]: cnn = psycopg2.connect("")
>      In [3]: cur = cnn.cursor()
>      In [4]: cur.execute("copy (select 1 as x) to stdout")
>      
> ---------------------------------------------------------------------------
>      ProgrammingError                          Traceback (most recent 
> call last)
>      <ipython-input-4-62e3af3d121e> in <module>()
>      ----> 1 cur.execute("copy (select 1 as x) to stdout")
>      ProgrammingError: can't execute COPY TO: use the copy_to() method 
> instead
> 
> but:
> 
>      In [6]: cnn = psycopg2.connect("", async=True)
>      In [7]: cnn.poll()
>      Out[7]: 2
>      In [8]: cnn.poll()
>      Out[8]: 1
>      In [9]: cnn.poll()
>      Out[9]: 0
> 
>      In [12]: cur.execute("copy (select 1 as x) to stdout")
>      In [14]: cnn.poll()
>      ^C -- hung
> 
> So yes, I'll take a look if there is a way to detect we get in this 
> state. But I'm not sure it's possible, because we might not have 
> received a result yet that tells we are in a state not allowed (in a COPY).
> 
> Observation: maybe EnterpriseDB has resources enough to help us 
> debugging and fixing this issue, or maybe even implement async copy?

Also, let me add that if the only way to detect the situation is by 
analyzing the query, we'll never do that.

Also, IMHO, it is perfectly fine that a sequence of operations that is 
documented as "not working" hangs - it is not something you can do by 
error: you're ignoring the documentation.

federico

-- 
Federico Di Gregorio                         federico.digregorio@dndg.it
DNDG srl                                                  http://dndg.it
    I filosofi son come i sociologi: il mondo non lo capiscono. -- A.R.M.