[pgpool-general: 968] Re: using prepared statements when memory_cache_enabled=on

[Lonni J Friedman]

Its somewhat encouraging knowing its not just me.  Are you seeing the
same error that I'm seeing?


On Thu, Sep 6, 2012 at 6:53 AM, Greg Swallow <gswallow at exacttarget.com> wrote:
> 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?