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

There is jxdbcon Postgresql jdbc driver with setFetchSize method.
Last version don't wokr with pgsql 7.3 and I don't test more.
I will try next day, when I download pgsql 7.2

regards
Haris Peco
On Friday 11 October 2002 07:59 pm, Dave Cramer wrote:
> Agreed, but there are selects where count(*) won't work. Even so, what
> we are talking about here is hiding the implementation of cursors behind
> the result set. What I would envision is some sort of cacheing where
> when the user set's the fetchsize to 10 for instance we do the select,
> and when they ask for next() we check to see if we have these rows in
> the cache, and go get them if necessary 10 at a time, possibly keeping
> one set of ten behind where we are and one set of 10 ahead of where we
> are. So recalling that resultSets have absolute positioning, as well as
> first(), and last() positioning we need the ability to move with the
> minimum number of trips to the backend.
>
> As it turns out the move command in postgres does support moving to the
> end (move 0 ); at the moment this is considered a bug, and is on the
> todo list to be removed. I expect we can get some sort of implementation
> which allows us to move to the end ( move end )
>
> Dave
>
> On Fri, 2002-10-11 at 13:12, Dror Matalon wrote:
> > Hi,
> >
> > I'm jumping in late into this discussion but ...
> >
> > In my mind a lot of these features break the model. From an application
> > prespective, if I want to do last, I do a count(*) and then I do a fetch
> > with limit; Not quite the same, but all these methods of fetching the
> > whole data locally and manipulating it to a large exten defeat the
> > purpose. Let the backend do the work, instead of trying to replicate the
> > functionality in JDBC.
> >
> > That said I do understand that some of these are required by the JDBC 2.0
> > spec.
> >
> > Dror
> >
> > On Fri, Oct 11, 2002 at 01:05:37PM -0400, Aaron Mulder wrote:
> > >     It wouldn't be bad to start with a naive implementation of
> > > last()...  If the only problem we have is that last() doesn't perform
> > > well, we're probably making good progress.  :)
> > >     On the other hand, I would think the updateable result sets would
> > > be the most challenging; does the server provide any analogous features
> > > with its cursors?
> > >
> > > Aaron
> > >
> > > On 11 Oct 2002, 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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> > --
> > Dror Matalon
> > Zapatec Inc
> > 1700 MLK Way
> > Berkeley, CA 94709
> > http://www.zapatec.com
> >
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