Message-ID: <44B51896.1000808@modgraph-usa.com>
Date: Wed, 12 Jul 2006 08:43:18 -0700
From: "Craig A. James" <cjames@modgraph-usa.com>
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To: Magnus Hagander <mha@sollentuna.net>
CC: pgsql-performance@postgresql.org
Subject: Re: Kill a session
References: <6BCB9D8A16AC4241919521715F4D8BCEA0FAE8@algol.sollentuna.se>
In-Reply-To: <6BCB9D8A16AC4241919521715F4D8BCEA0FAE8@algol.sollentuna.se>
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Magnus Hagander wrote:
>> This raises the question: Why doesn't Postgres have a "kill 
>> session" command that works?  Oracle has it, and it's 
>> invaluable; there is no substitute.  Various writers to these 
>> PG lists have raised the question repeatedly.  Is it just a 
>> matter that nobody has had the time to do it (which I 
>> respect!), or is there a reason why the Postgres team decided 
>> a "kill session" is a bad idea?
> 
> I beleive the function to kill a backend is actually in the codebase,
> it's just commented out because it's considered dangerous. There are
> some possible issues (see -hackers archives) about sending SIGTERM
> without actually shutting down the whole cluster.
> 
> Doing the client-side function to call is the easy part.
> 
> In many cases you just need to cancel a query, in which case you can use
> pg_cancel_backend() for exmaple. If you need to actually kill it, your
> only supported way is to restart postgresql. 

In other words, are you confirming that there is no way to kill a query from another process, other than shutting down the database?  My understanding of the documentation tells me I can't use cancel, because the process doing the killing isn't the original process.

>> But in spite earlier posting in these forums that say the 
>> killing the backend was the way to go, this doesn't really 
>> work.  First, even though the "postgres" backend job is 
>> properly killed, a "postmaster" job keeps running at 99% CPU, 
>> which is pretty useless.  Killing the client's backend didn't 
>> kill the process actually doing the work!
> 
> Then you killed the wrong backend...
> No queries run in postmaster. They all run in postgres backends. The
> postmaster does very little actual work, other than keeping track of
> everybody else.

It turns out I was confused by this: ps(1) reports a process called "postgres", but top(1) reports a process called "postmaster", but they both have the same pid.  I guess postmaster replaces its own name in the process table when it's executing a query, and it's not really the postmaster even though top(1) calls it postmaster.

So "kill -15 <pid>" is NOT killing the process -- to kill the process, I have to use signal 9.  But if I do that, ALL queries in progress are aborted.  I might as well shut down and restart the database, which is an unacceptable solution for a web site.

I'm back to my original question: How do you kill a runaway query without bringing down the whole database?  Is there really no answer to this?

Thanks,
Craig