Fwd: [HACKERS] client performance v.s. server statistics

Hi,

Forward my question from HACKERS list to here (and added some more notes):

I have tried unix domain socket and the performance is similar with
TCP socket. It is MIPS architecture so memory copy to/from kernel can
occupy much time, and apparently using unit domain socket has no
difference than TCP in terms of memory copy.

But it is still unbelievable for the ten-fold gap between the client
side statistic and the server side statistics. So I want to know what
exactly the operations are involved in the server side statistics in
EXPLAIN ANALYZE. May I check the code later on when I get time.

For the query itself, it was just for performance comparison. There
are other index based queries, which are of course much faster, but
still result in similar ten-fold of time gap between client side and
server side statistics.

I am thinking of non-kernel involved client interface, is there such
an option, or do I have to develop one from scratch?

Besides, the test was done on the same host (without network cost).
And even considering the memory copying cost it is not reasonable,
because the client did similar job using another IPC mechanism via
kernel space to transfer the data again to another program, which
appears to be quite fast - costed even much less than the time shown
by EXPLAIN ANALYZE.

Is there anyone can help me to explain this?

Best regards,
Han

On Wed, Feb 15, 2012 at 1:23 PM, Amit Kapila <amit(dot)kapila(at)huawei(dot)com> wrote:
>
> >>So, is it client interface (ODBC, libpq) 's cost mainly due to TCP?
>
>
>
> The difference as compare to your embedded DB you are seeing is mainly seems to be due to TCP.
>
> One optimization you can use is to use Unix-domain socket mode of PostgreSQL. You can refer unix_socket_directory parameter in postgresql.conf and other related parameters.
>
> I am suggesting you this as earlier you were using embedded DB, so your client/server should be on same machine. If now this is not the case then it will not work.
>
>
>
> Can you please clarify some more things like
>
> 1.      After doing sequence scan, do you need all the records in client for which seq. scan is happening. If less records then why you have not created index.
>
> 2.      What is exact scenario for fetching records
>
>
>
>
>
>
>
> pgsql-hackers-owner(at)postgresql(dot)org [mailto:pgsql-hackers-owner(at)postgresql(dot)org] On Behalf Of Zhou Han
> Sent: Wednesday, February 15, 2012 9:30 AM
> To: pgsql-hackers(at)postgresql(dot)org
> Subject: [HACKERS] client performance v.s. server statistics
>
>
>
> Hi,
>
> I am checking a performance problem encountered after porting old embeded DB to postgreSQL. While the system is real-time sensitive, we are concerning for per-query cost. In our environment sequential scanning (select * from ...) for a table with tens of thousands of record costs 1 - 2 seconds, regardless of using ODBC driver or the "timing" result shown in psql client (which in turn, relies on libpq). However, using EXPLAIN ANALYZE, or checking the statistics in pg_stat_statement view, the query costs only less than 100ms.
> rface (ODBC, libpq) 's cost mainly due to TCP? Has the pg_stat_statement or EXPLAIN ANALYZE included the cost of copying tuples from shared buffers to result sets?
>
> Could you experts share your views on this big gap? And any suggestions to optimise?
>
> P.S. In our original embeded DB a "fastpath" interface is provided to read directly from shared memory for the records, thus provides extremely realtime access (of course sacrifice some other features such as consistency).
>
> Best regards,
> Han

--
Best regards,
Han