[Haskell-cafe] Re: Greetings

[Krasimir Angelov]

On 10/1/06, Seth Gordon <sethg at ropine.com> wrote:
> Paul Johnson wrote:
> > I've done some stuff with maybe 50k rows at a time.  A few bits and pieces:
> >
> > 1: I've used HSQL
> > (http://sourceforge.net/project/showfiles.php?group_id=65248) to talk to
> > ODBC databases.  Works fine, but possibly a bit slowly.  I'm not sure
> > where the delay is: it might just be the network I was running it over.
> > One gotcha: the field function takes a field name, but its not random
> > access.  Access the fields in query order or it crashes.
>
> Thanks; that's certainly the sort of thing I like knowing in advance.

This behaviour depends on the underlying database. I remember that
MSSQL suffers from this disease. In addition with MSSQL you can't have
more than one opened dataset for a given connection. MySQL and
PostgreSQL doesn't have this problem. HSQL can't hide all differences
between the possible backends.

> > 2: For large data sets laziness is your friend.  When reading files
> > "getContents" presents an entire file as a list, but its really
> > evaluated lazily.  This is implemented using unsafeInterleaveIO.  I've
> > never used this, but in theory you should be able to set up a query that
> > returns the entire database as a list and then step through it using
> > lazy evaluation in the same way.
>
> I assume that the collectRows function in HSQL can produce this kind of
> a lazy list...right?

No. collectRows collects all records eagerly. The problem with the
lazy fetching is that you can close the database connection while your
lazy data sets aren't fetched yet. It is getting worse if you can't
have multiple opened data sets. If you still want lazy fetching you
can write a custom function like collectRows but using
unsafeInterleaveIO.


Cheers,
  Krasimir