[pgpool-general: 1057] Re: using prepared statements when memory_cache_enabled=on

[Lonni J Friedman]

On Sat, Sep 29, 2012 at 5:07 AM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>> On Sun, Sep 16, 2012 at 12:18 AM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>>>>> On Fri, Sep 14, 2012 at 3:37 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>>>>>>> On Fri, Sep 14, 2012 at 3:09 PM, Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
>>>>>>>>> On Thu, Sep 13, 2012 at 6:35 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>>>>>>>>>>> pg_* functions.  Stuff like pg_prepare, pg_exec, pg_query.
>>>>>>>>>>
>>>>>>>>>> Can you show us pg_prepare example? I'm interested in what statement
>>>>>>>>>> names you are using.
>>>>>>>>>
>>>>>>>>> pg_prepare($db, $preparedStatementKey, "SELECT
>>>>>>>>> id,pass,fail,filtercount,info,current_status,last_update,os,arch,branch,gpu,build_type,subtest,osversion
>>>>>>>>> FROM cudasmoke WHERE (last_update > $1 AND last_update < $2) AND os=$3
>>>>>>>>> AND arch=$4 AND branch=$5 AND build_type= $6 AND subtest=$7 AND
>>>>>>>>> osversion=$8 AND gpu=$9 ORDER BY last_update DESC LIMIT 1" )
>>>>>>>>
>>>>>>>> So what's the content of $preparedStatementKey?
>>>>>>>
>>>>>>> Its a unique identifier string for the prepared statement, comprised
>>>>>>> of the concatenated sha1sum and md5sum of the prepared statement.
>>>>>>>
>>>>>>> So it ends up looking something like this:
>>>>>>> pg_execute($db,a1a3a2f461a0efc2eee2f60f940df6b77eb552f9f5a985165c3ab1665384fa9cce8b1b62,"SELECT
>>>>>>> id,pass,fail,filtercount,info,current_status,last_update,os,arch,branch,gpu,build_type,subtest,osversion
>>>>>>> FROM cudasmoke WHERE (last_update > $1 AND last_update < $2) AND os=$3
>>>>>>> AND arch=$4 AND branch=$5 AND build_type= $6 AND subtest=$7 AND
>>>>>>> osversion=$8 AND gpu=$9 ORDER BY last_update DESC LIMIT 1);
>>>>>>
>>>>>> The statement name is 72 byte long. This is interesting because
>>>>>> PostgreSQL (and pgpool) does not suppose that statement name is longer
>>>>>> than 64.  Not sure how this affects to the problem though.
>>>>>>
>>>>>> src/include/pg_config_manual.h:
>>>>>> /*
>>>>>>  * Maximum length for identifiers (e.g. table names, column names,
>>>>>>  * function names).  Names actually are limited to one less byte than this,
>>>>>>  * because the length must include a trailing zero byte.
>>>>>>  *
>>>>>>  * Changing this requires an initdb.
>>>>>>  */
>>>>>> #define NAMEDATALEN 64
>>>>>
>>>>> That is interesting, and I was unaware of this limitation.  Anyway,
>>>>> even if, for the purposes of experimentation, I only use the sha1sum
>>>>> (and drop the md5sum) such that the identifier is reduced in size to
>>>>> 40 bytes, I still see the same failure.  However, one thing that I
>>>>> noticed is that pgpool doesn't seem to realize that I've attempted to
>>>>> use a new prepared statement identifier (now just the sha1sum, without
>>>>> md5sum), and still tries to use the previously cached cached one.
>>>>>
>>>>> I'd expect that if I'm preparing a statement with an unused (new)
>>>>> identifier, it shouldn't pull it from the cache, but it appears to do
>>>>> so anyway.
>>>>
>>>> Why do you think so? If SELECT(and with same parameters) is identical,
>>>> it should return exactly same results regardress the statement name(of
>>>> courese I do not account underlying tables get modified).
>>>
>>> BTW, we have recently fixed a bug which can cause buffer overrun error:
>>> http://git.postgresql.org/gitweb/?p=pgpool2.git;a=commit;h=3964d8204373473e3130a9ba5b260049420dafb6
>>>
>>> Because it's a memory destruction error, it may or may not be related to your problem. Can you try with pgpool-II 3.2-stable head?
>>
>> ok, I've done that now, but the same problem is still present.
>
> I seem to find the cause of problem and created patch. Please take a
> look at bugtrack #21.
>
> http://www.pgpool.net/mantisbt/view.php?id=21
>
> The patch is here:
> http://www.pgpool.net/mantisbt/file_download.php?file_id=28&type=bug

I applied the patch, rebuilt everything, enabled memcache, restarted
pgpool, and retested.  Unfortunately, nothing has changed or improved,
and the same error persists.