[odb-users] Retrieving Results from Native Queries without Compile-time Fixed Classes

[Boris Kolpackov]

ATSDB <atsdb at gmx.at> writes:

> The short question is: I would like to use native queries to obtain
> partial data (some columns of many) from existing (unmapped) tables
> without having to have compile-time fixed classes to hold the results.
> Is this possible?

While it should be possible using implementation details (basically
what the generated code is using), this is not a documented API, there
are no stability guarantees, etc. You will also have to write this
code for each database that you want to support.

Here is, for example, select_statement for SQLite:

https://git.codesynthesis.com/cgit/odb/libodb-sqlite/tree/odb/sqlite/statement.hxx#n210


> From my understanding this is somewhat supported using Views (as stated
> in the odb manual), but only if I write result classes that store the
> selected columns - which is then compile-time fixed. Is my understanding
> correct?

It is.


> Background: My applications stores large amounts of data in a database
> (gigabytes). I wrote a (simplistic) system which allows data abstraction
> from the actual database schema based on configuration. During loading, the
> current schema is used to dynamically create the select queries with
> not-fixed number/names of columns, and the result is stored in custom
> container (basically a dynamic templated aggregations of std::vectors -
> think array of structs with dynamic amount of members).

I assume the types of these columns are not known at compile time either?


> Would it be possible/reasonable to use mapped classes and additionally
> directly use the native database connections to retrieve the results
> to fill my custom containers?

I think it would be reasonable if you don't have a better alternative.
One thing going for ODB here (IMO), is that the database runtimes are
pretty efficient and well-tested.