Re: Retrieving ResultSets with millions of rows - - Mailing list pgsql-jdbc

On Fri, 2002-09-13 at 20:45, Doug Fields wrote:
>
> >Ya, there is, it's a bit of a hack but the only way around it is to use
> >a cursor. Eventually the driver will do this automatically, but for now
> >you will have to do it manually
>
> That's what I was afraid you were going to say. Thanks for the response.
>
> Question: Do I have to do anything unusual with the JDBC Connection and/or
> Statement to put it in a certain mode? I'm not much on the interactions
> between the JDBC API and the database transactions (begin/commit, etc.). I
> also use the Protomatter connection pool; what do I have to do with the
> connection when I release it back to the pool?
yes, turn autocommit off, and then to be safe turn it back on before
returning it to the pool.
>
> Also, a slightly tangential question: If I'm only doing reads in the
> transaction (the cursor stuff), is there any performance difference if I
> COMMIT or ROLLBACK?
with autocommit off you don't have to do either, just do an end;
>
> Thanks,
>
> Doug
>
>
> >so
> >begin;
> >declare cursor1 CURSOR FOR SELECT ....
> >fetch n from cursor1 ...
> >end;
> >
> >Dave
> >On Fri, 2002-09-13 at 17:43, Doug Fields wrote:
> > > Hello,
> > >
> > > I've just come to the startling realization that the 7.2 JDBC code loads
> > > every single row in a ResultSet into a Vector. (core/QueryExecutor.java)
> > >
> > > Unfortunately, I run some queries which return millions of rows, each row
> > > which could well be 100-1000 bytes and more.
> > >
> > > Hence, I get an OutOfMemoryError.
> > >
> > > For some queries, there's an obvious workaround: issue the query with an
> > > appropriate suffix "ORDER BY <something> LIMIT <something> OFFSET
> > > <something>" several times in succession. This will, of course, work
> > mostly
> > > fine assuming it's a simple single-table query with an appropriate index
> > > (such as a primary key).
> > >
> > > However, some of my queries are complex one-to-many joins with no
> > > particular ordering (or no index for quick ordering). These would be much
> > > harder to do that trick with, and/or incur extreme amounts of database
> > > overhead in running the query hundreds of times (for example, if I were to
> > > handle 10,000 rows at a time).
> > >
> > > Is there any way to get a ResultSet which is actually live streamed, which
> > > keeps no knowledge of previous rows or the row count? It seems utterly
> > > wasteful to me that I should need to load millions of rows into a Vector
> > > (try an ArrayList next time, for minor speedups) when all I want to do is
> > > stream the results, not ever needing to know the total number of rows
> > ahead
> > > of time nor any previous (or subsequent) rows to the "current one" from
> > > ResultSet.next().
> > >
> > > I can't imagine I'm the first person to be using JDBC to access tables
> > with
> > > tens of millions of rows to attempt to access significant portions of them
> > > at a time.
> > >
> > > Thanks,
> > >
> > > Doug
> > >
> > >
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>
>
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