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From: Corwin Joy (cjoy_at_[hidden])
Date: 2005-08-23 19:44:56

----- Original Message -----
From: "Brock Peabody" <brock.peabody_at_[hidden]>

> Hi Corwin,
> Do you think our interface should support bi-directional iteration to
> avoid the pitfalls you mention with input iterators, or should we only
> support input iterators to support the broadest number of
> implementations?

I think we should support both an input iterator and a random access
The reason for this is that, as mentioned, for many databases by far the
most efficient
operation is to just retrieve the set of records via a forward-only cursor
and not
provide any guarantee that the order of rows is repeatable. As soon as you
have to
maintain a specific order for the records (which is what a bi-directional
iterator implies)
then the database often has to take a huge performance hit. As mentioned,
drivers implement this by downloading all records to the local computer
locking the entire table or other ugly things. Hopefully the driver is
smart and can do
version tracking but it is hard to be sure. That, to me, is why logically
you have two
C++ iterator types that make sense: an input iterator and a random access

> Do you know of any databases whose native APIs don't
> support bi-directional iteration?

Well, for example MS Access used to not support this via ODBC, but even if
they do support it I don't think it is a good idea to *require* it for the
iterator since
this can have big performance costs. Also there are some kinds of result
sets which may be
forward only by definition. Result sets from stored procedures come to mind
> If you get a chance you might check out the project in
> boost-sandbox/database.

I've browsed through it somewhat online. I need to look through the boost
site docs
and find out how to connect up to the sandbox via CVS.

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