Re: [JDBC] Out of memory error on huge resultset - Mailing list pgsql-hackers

Hello,
  Does it mean that psql uses cursors ?

regards
Haris Peco
On Friday 11 October 2002 05:58 pm, Dave Cramer wrote:
> This really is an artifact of the way that postgres gives us the data.
>
> When you query the backend you get *all* of the results in the query,
> and there is no indication of how many results you are going to get. In
> simple selects it would be possible to get some idea by using
> count(field), but this wouldn't work nearly enough times to make it
> useful. So that leaves us with using cursors, which still won't tell you
> how many rows you are getting back, but at least you won't have the
> memory problems.
>
> This approach is far from trivial which is why it hasn't been
> implemented as of yet, keep in mind that result sets support things like
> move(n), first(), last(), the last of which will be the trickiest. Not
> to mention updateable result sets.
>
> As it turns out there is a mechanism to get to the end move 0 in
> 'cursor', which currently is being considered a bug.
>
> Dave
>
> On Fri, 2002-10-11 at 11:44, Doug Fields wrote:
> > At 08:27 AM 10/11/2002, snpe wrote:
> > >Barry,
> > >   Is it true ?
> > >I create table with one column varchar(500) and enter 1 milion rows with
> > >length 10-20 character.JDBC query 'select * from a' get error 'out of
> > >memory', but psql not.
> > >I insert 8 milion rows and psql work fine yet (slow, but work)
> >
> > The way the code works in JDBC is, in my opinion, a little poor but
> > possibly mandated by JDBC design specs.
> >
> > It reads the entire result set from the database backend and caches it in
> > a horrible Vector (which should really be a List and which should at
> > least make an attempt to get the # of rows ahead of time to avoid all the
> > resizing problems).
> >
> > Then, it doles it out from memory as you go through the ResultSet with
> > the next() method.
> >
> > I would have hoped (but was wrong) that it streamed - WITHOUT LOADING THE
> > WHOLE THING - through the result set as each row is returned from the
> > backend, thus ensuring that you never use much more memory than one line.
> > EVEN IF you have to keep the connection locked.
> >
> > The latter is what I expected it to do. The former is what it does. So,
> > it necessitates you creating EVERY SELECT query which you think has more
> > than a few rows (or which you think COULD have more than a few rows,
> > "few" being defined by our VM memory limits) into a cursor based query.
> > Really klugy. I intend to write a class to do that for every SELECT query
> > for me automatically.
> >
> > Cheers,
> >
> > Doug
> >
> > >In C library is 'execute query' without fetch - in jdbc execute fetch
> > > all rows
> > >and this is problem - I think that executequery must prepare query and
> > > fetch (ResultSet.next or ...) must fetch only fetchSize rows.
> > >I am not sure, but I think that is problem with jdbc, not postgresql
> > >Hackers ?
> > >Does psql fetch all rows and if not how many  ?
> > >Can I change fetch size in psql ?
> > >CURSOR , FETCH and MOVE isn't solution.
> > >If I use jdbc in third-party IDE, I can't force this solution
> > >
> > >regards
> > >
> > >On Thursday 10 October 2002 06:40 pm, Barry Lind wrote:
> > > > Nick,
> > > >
> > > > This has been discussed before on this list many times.  But the
> > > > short answer is that that is how the postgres server handles queries.
> > > >  If you issue a query the server will return the entire result.  (try
> > > > the same query in psql and you will have the same problem).  To work
> > > > around this you can use explicit cursors (see the DECLARE CURSOR,
> > > > FETCH, and MOVE sql commands for postgres).
> > > >
> > > > thanks,
> > > > --Barry
> > > >
> > > > Nick Fankhauser wrote:
> > > > > I'm selecting a huge ResultSet from our database- about one million
> > > > > rows, with one of the fields being varchar(500). I get an out of
> > > > > memory error from java.
> > > > >
> > > > > If the whole ResultSet gets stashed in memory, this isn't really
> > > > > surprising, but I'm wondering why this happens (if it does), rather
> > > > > than a subset around the current record being cached and other rows
> > > > > being retrieved as needed.
> > > > >
> > > > > If it turns out that there are good reasons for it to all be in
> > > > > memory, then my question is whether there is a better approach that
> > > > > people typically use in this situation. For now, I'm simply
> > > > > breaking up the select into smaller chunks, but that approach won't
> > > > > be satisfactory in the long run.
> > > > >
> > > > > Thanks
> > > > >
> > > > > -Nick
> > > > >
> > > > > -------------------------------------------------------------------
> > > > >------ - Nick Fankhauser  nickf@ontko.com  Phone 1.765.935.4283  Fax
> > > > > 1.765.962.9788 Ray Ontko & Co.     Software Consulting Services
> > > > > http://www.ontko.com/
> > > > >
> > > > >
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