[pgpool-general: 8402] Re: Extended query and cache

[Tatsuo Ishii]

> Hi,
> I am not sure I understood what you mean when you mentioned “close message”
> 
> But i’ll try to ask you in a different way.
> 
> Let look on the following example:
> 
> 1. Select * from users where id = $1; using extended query. —> miss
> 
> 2. Now in case i’ll run the same query i’ll get it back from cache.
> 
> So I am understanding why the parse message should be pass to the backend
> and not read the result from cache due to that Parse message didn’t
> contains the query params. That’s make sense.
> 
> But in Bind message the packet already contains the query params. So why
> you can’t read the result from cache here and we need to wait to the
> execute message?

Bind message does not return the "result". It just returns it succeeded
(bind complete message) or failed (error response). The actual result
(which is called "portal") is only in PostgreSQL's memory. So "caching
result of bind message" is almost meaningless.

> One more question is why we need to send the bind message anyway to the
> backend in case we have already the result of the execute message in cache?

Think about this scenario:

1. pgpool receives a bind message but finds that the corresponding
query cache exists. So pgpool does not send the bind message to
backend.

2. In other session the table used in the query has been modified and
the query is gone.

3. pgpool receives an execute message and tries to extract the query
cache, but it has already gone. However pgpool cannot send the execute
message to backend because the result of bind message (portal) does
not exist in the backend.

> I am asking the above questions because I am looking in the rds performance
> insights.
> And I can see that in case I am sending a lot of queries concurrently when
> I have cache enabled I can see a very big wait because of a lot of
> connections in idle ClientRead wait event.

I am not familiar with performance insights and cannot comment on it.

> And I am asking myself why the db needs to know about queries we already
> have in cache?
> 
> Hope it was clear.
> 
> 
> Thanks a lot,
> 
> Avi
> 
> 
> 
> On Wed, 14 Sep 2022 at 2:42 Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
> 
>> > Hi,
>> >
>> > In case query already cached, why in extended query mode the db knows
>> about
>> > that query?
>> > Because in that case pgpool should return the result from the cache in
>> > order to save db resources.
>> > But I found that although we have the query in cache pgpool still send
>> > parse and bind request to the db.
>> >
>> > Please share your thoughts 🙏
>>
>> Yes, Pgpool-II only caches the result of execute message. The main
>> reason is, to not return stale cache.  After receiving a close message
>> for the statement or the portal that is bound to the execute message,
>> the cache for the execute message should not be returned. When
>> Pgpool-II receives close messages, they remove the internal record of
>> previpusly received statement or portal. When an execute message
>> arrives, Pgpool-II checks whether the record for the execute message
>> exists. If does not, the request fails. This strategy is simple but
>> works well.
>>
>> I think the saving by caching parse message is not small for
>> especially complex queries. But in this case users already reuse the
>> prepared statement anyway.
>>
>> I think the saving of bind message is usually small because it does
>> not involve planning in most cases.
>>
>> Best reagards,
>> --
>> Tatsuo Ishii
>> SRA OSS LLC
>> English: http://www.sraoss.co.jp/index_en/
>> Japanese:http://www.sraoss.co.jp
>>