Re: [htoolkit-users] HSQL, HList

[Bjorn B. <d00...@dt...>]

Frederik Eaton wrote:
>>You might want to be careful with collectRowsAsync. We have the same=20
>>thing in HaskellDB, and it turns out to be a problem with some database=
s=20
>>if you run a second query without getting all that data from the first=20
>>one. I think MySQL with the native drivers does it for example, but not=
=20
>>with ODBC.
>=20
> What are the symptoms?

If you already have an open result and try to run another query, you=20
will get an expcetion with the MySQL drivers.

> What I'm seeing is that if I run closeStatement before reading all the
> rows from a query, it goes ahead and reads the rest of the rows
> silently, which is bad if there are millions of rows. For instance, if
> there is an error, like if I request a field with the wrong type, then
> since these are caught by a 'finally' clause in getFieldValue and
> trigger a closeStatement, the error message isn't printed (by
> 'handleSql print' in main) until the whole table is read in, several
> minutes later.

That doesn't seem like what you'd expect. Bug in the driver, or HSQL mayb=
e?

> I'm trying to get MyODBC installed - this is necessary using MySQL
> through ODBC, right? - to see if the behavior is any different, since
> you seemed to imply it would be less problematic.

Well, yes and no. You don't get the exception like with the MySQL=20
driver, but the reason for that is that the MyODBC driver seems to fetch=20
the entire result set from the server straight away and keep it locally.=20
So you can have two open queries, but the only thing you gain from=20
laziness is that the data you never use will be in C code, not Haskell,=20
and thus take up less space. But that of course won't help if the=20
results are larger that you local RAM for example.

/Bj=F6rn