Re: One source of constant annoyance identified - Mailing list pgsql-general

Hello!

Thank you very much for your efforts - we appreciate that very much :)

> -----Ursprüngliche Nachricht-----
> Von: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Gesendet: Mittwoch, 3. Juli 2002 00:28
> An: Markus Wollny
> Cc: pgsql-general@postgresql.org
> Betreff: Re: [GENERAL] One source of constant annoyance identified
>
>
> "Markus Wollny" <Markus.Wollny@computec.de> writes:
> > Sorry I took so long - I attached the schema as asked.
>
> Thanks.  But I'm still unable to reproduce the memory bloat you see on
> SELECTs.  This seems very peculiar.  You said you were
> running SuSE 7.3
> --- how recent is that?  Which glibc version is it running?
> (I've been
> reduced to speculating about memory leakage inside libc, which is a
> pretty long shot but...)

I agree - it is a long shot: SuSE 7.3 has got Kernel: 2.4.10 and glibc:
2.2.4; it was released in October 2001. I tried using SuSE 8.0, released
in late April this year, but I was more than unhappy with some of the
new "features" - and it seems that hardly anybody has switched to 8.0
for server-usage as yet. Generally SuSE 7.3 (which is probably by far
the most popular distro in Germany) is considered quite stable and
current enough for server-usage. Would it really be worth the hassle
updating glibc to 2.2.5?

> > I took a quick look at top: Even this humble query causes
> memory- and
> > processor-load like a giant: 266M RAM, 38.3% processor time, 26.4%
> > memory usage. Okay, it's calling the trigger for each row
> which in turn
> > inserts some new tuples into ct_com_board_fti, but is it expected to
> > cause so much load?
>
> Wouldn't surprise me.  Since you're using an AFTER trigger,
> the pending
> trigger events have to be saved up for commit time, so the list of
> pending events is going to grow quite large.

Okay, so there are indeed situations when this kind of backend size can
be considered normal. That's some sort of relief :)

> (How many rows do you have  in ct_com_board_message, anyway?
> How many did that query try to
> update?)  This however does not explain your problem with
> SELECT, since
> selects don't fire triggers.

Currently there are 362,154 rows in ct_com_board_message and 85,101 rows
in ct_com_user. I don't know if this can be considered a lot; we will
expect that to grow at an accelerating rate during the next months, so
more than a million can be expected within the next 12 months or so. We
scarcely use any database-specific features like triggers as yet, it's
99.99% pure SQL, tables, indexes, data, selects, inserts, updates,
deletes, no "fancy stuff" as yet. We'll have to get to know PostgreSQL
better before using anything specific; we never needed to get too deep
into Oracle either, which was an advantage when porting the whole thing
over to PostgreSQL, but it seems that the latter calls for a bit more
attention and know-how...

> Could I see the output of EXPLAIN for that problem SELECT on your
> machine?

We are currently working on a new version for the search I gave you
before, but here's another one that's causing similar symptoms - extra
long running time and most probably huge backends (it's 10:20 am, far
from peak time, and we already have ~280MB swap and six backends
>100MB):

This one generates an overview over all the forum-threads in one board
which has 41,624 messages, 2971 of them are FATHER_ID=0, so
thread-starters:

select          MESSAGE.BOARD_ID
                    , MESSAGE.THREAD_ID
                    , MESSAGE.FATHER_ID
                    , MESSAGE.MESSAGE_ID
                    , MESSAGE.USER_ID
                    , MESSAGE.USER_LOGIN
as LOGIN
                    , MESSAGE.USER_STATUS
as STATUS
                    , MESSAGE.USER_RIGHTS
as RIGHTS
                    , MESSAGE.TITLE
                    , MESSAGE.COUNT_REPLY
as COUNT_REPLY
                    ,
to_char(MESSAGE.LAST_REPLY,'DD.MM.YY hh24:mi')    as LAST_REPLY
                    ,
round((date_part('epoch',CURRENT_TIMESTAMP)-date_part('epoch',MESSAGE.LA
ST_REPLY))/60)      as diff_posting
                    ,
to_char(MESSAGE.CREATED,'DD.MM.YY hh24:mi')        as DATUM

                    , (select count(*) from
CT_COM_USER_THREAD_FOLLOW where USER_ID= '295798' and
thread_id=MESSAGE.THREAD_ID) as TFUID

        from          CT_COM_BOARD_MESSAGE    MESSAGE
        where        (0=0)

        and            MESSAGE.BOARD_ID    = 10
        and            MESSAGE.FATHER_ID    = 0
        and            MESSAGE.STATE_ID    = 0
        order by    MESSAGE.LAST_REPLY    desc


Sort  (cost=30695.27..30695.27 rows=7693 width=154) (actual
time=9745.94..9751.58 rows=4663 loops=1)
  ->  Index Scan using idx_bm_show_topics on ct_com_board_message
message  (cost=0.00..30198.72 rows=7693 width=154) (actual
time=111.56..9549.99 rows=4663 loops=1)
        SubPlan
          ->  Aggregate  (cost=5.83..5.83 rows=1 width=0) (actual
time=0.91..0.91 rows=1 loops=4663)
                ->  Index Scan using idx_user_thread_follow on
ct_com_user_thread_follow  (cost=0.00..5.83 rows=1 width=0) (actual
time=0.88..0.88 rows=0 loops=4663)
Total runtime: 9835.57 msec

I do hope you can make anything of this...

Regards,

    Markus