[pgpool-general: 966] Re: using prepared statements when memory_cache_enabled=on

Greg Swallow gswallow at exacttarget.com
Thu Sep 6 22:53:26 JST 2012

I'm in the same boat, with a Rails app.  I can't reproduce it running the same query in psql, but once I turned the memory cache off, I stopped having this issue. I'm using membase and moxi, rather than memcached.  It's wire-format compatible from what I can tell.  PostgreSQL 9.1.4.  I can possibly help debug.

On Sep 5, 2012, at 5:26 PM, Lonni J Friedman wrote:

> On Tue, Sep 4, 2012 at 9:30 PM, Nozomi Anzai <anzai at sraoss.co.jp> wrote:
>> Hi,
>>> Greetings,
>>> I'm running 3.2.0 with memory_cache_enabled=on and memqcache_method =
>>> 'memcached', and I'm seeing really bizarre errors when using prepared
>>> statements.  Basically any query that should use the prepared
>>> statement is failing with:
>>> server sent data ("D" message) without prior row description ("T" message)
>>> This only happens once the data has been cached by pgpool (with the
>>> memcache server).  If I set memory_cache_enabled=off, then it works
>>> fine.  I'm guessing that maybe the problem is that the prepared
>>> statement isn't getting cached, so the queries which depend on the
>>> prepared statement fail?
>>> Is this expected behavior?  If so, is there a work around that doesn't
>>> require me to not cache the tables that the prepared statement
>>> reference?
>> I couldn't reproduce it by the following ways:
>> 1) the attached simple Java program: pgsql.java (only executing one SELECT)
>> 2) pgbench -M extended -f bench.sql
>> Which version is your PostgreSQL? I tested with pgpool-II 3.2.0
>> (officially released), PostgreSQL 9.1.0, and memcached 1.4.7.
> postgresql-9.1.5
> memcached-1.4.10
> However, it looks like I was mistaken when I stated that this was
> triggered with prepared statements.  I tried to put together a trivial
> test case (similar to what you created), and now I can't replicate it.
> So whatever is going wrong only seems to happen inside of my huge,
> complex web app.    FWIW, I'm using PHP, not java.  All I know is that
> this reproduces 100% of the time whenever memory_cache_enabled=on, and
> never whenever memory_cache_enabled=off.
> I'm not sure how to debug this further.  Do you have any suggestions
> on anything that I can change with pgpool to better isolate what's
> going wrong?
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Greg Swallow
System Administrator
Phone | 317 524-5250
Yahoo IM | gswallow01
Email  | gswallow at exacttarget.com

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