[pgpool-general: 962] Re: using prepared statements when memory_cache_enabled=on

Lonni J Friedman netllama at gmail.com
Thu Sep 6 06:26:33 JST 2012

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.


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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