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Re: active queries

From: Alban Hertroys <dalroi(at)solfertje(dot)student(dot)utwente(dot)nl>
To: Miklosi Attila <amiklosi(at)freemail(dot)hu>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: active queries
Date: 2008-05-27 17:21:11
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
On May 27, 2008, at 3:08 PM, Miklosi Attila wrote:

> Hi!
> Our company has a long last problem by using libpq in multi-threaded
> programs. The libpq usually closes the programs without any error
> message or rarely giving the 'Invalid frontend message type 87' error.
> When asked you about this error message you gave the reply below.  
> In the
> reply there is only one thing which is not clear... What does an
> 'active' query mean? Or how can we detect active queries on a PGconn?

THE 'active' query (not AN) is the query that's currently being  
executed on a connection. There can be multiple connections with an  
active query each. You can't execute multiple queries in parallel on  
the same connection. You can execute them in sequence of course.

What people are trying to tell you is that you apparently have  
multiple threads that perform queries simultaneously on the same  
connection, and that's not possible.

You either need to put locks on the threads to prevent queries from  
happening simultaneously, or you should use a separate connection for  
each thread that executes queries in parallel.

To summarise, you need to serialise your queries on the connection or  
you need to parallelise your connections. Or a mix of both.

> We have tried PQcancel() and PQreset() functions too, but they didn't
> help solving the problem.
> Our programs retrieve every result (PQgetResult() and PQclear()
> functions) that is pending on the server before reusing a PGconn.
> Thank you in advance
> Attila Miklosi
> P.S.: this is a copy of your reply to 'invalid frontend message  
> type 87'
> error message
>> "Invalid frontend message" means the server got a message with an
>> unexpected first byte, which usually means the server and libpq got
>> out of sync about where the message boundaries are.  It's been quite
>> a long time since we've seen an actual bug of that sort, though.
>> The cases that I've heard of recently involve multiple threads in an
>> application trying to use the same PGconn without any interlocking.
>> You can't have multiple queries active on a single connection, but
>> libpq itself doesn't contain any locking to prevent multiple threads
>> from trying to use the PGconn at once.
>> If you're getting this with a single-threaded client, please submit
>> a test case to pgsql-bugs.
>>                       regards, tom lane
> -- 
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> To make changes to your subscription:

Alban Hertroys

If you can't see the forest for the trees,
cut the trees and you'll see there is no forest.


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