Re: Out of memory error on huge resultset - Mailing list pgsql-jdbc

No,

It doesn't have to store them, only display them

Dave
On Fri, 2002-10-11 at 12:48, snpe wrote:
> 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/
> > > > > >
> > > > > >
> > > > > > ---------------------------(end of
> > > > > > broadcast)--------------------------- TIP 5: Have you checked our
> > > > > > extensive FAQ?
> > > > > >
> > > > > > http://www.postgresql.org/users-lounge/docs/faq.html
> > > > >
> > > > > ---------------------------(end of
> > > > > broadcast)--------------------------- TIP 6: Have you searched our
> > > > > list archives?
> > > > >
> > > > > http://archives.postgresql.org
> > > >
> > > >---------------------------(end of broadcast)---------------------------
> > > >TIP 2: you can get off all lists at once with the unregister command
> > > >     (send "unregister YourEmailAddressHere" to
> > > > majordomo@postgresql.org)
> > >
> > > ---------------------------(end of broadcast)---------------------------
> > > TIP 6: Have you searched our list archives?
> > >
> > > http://archives.postgresql.org
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 5: Have you checked our extensive FAQ?
>
> http://www.postgresql.org/users-lounge/docs/faq.html
>
>