[OJB-developers] "inheritance mapping on one table" performance

[Matt G. <mat...@ss...>]

Hi all,

I've got a few more comments on the "inheritance mapping on one table"
scenario ...

1. Extra objects are created

When retrieving all records of a derived class creates an object for *every*
record in the table regardless of whether the ojbConcreteClass matches the
type (or derived type) requested. Obviously, that has a huge performance
impact if the table is large.

I tried implementing a RowReader and returning a null class descriptor when
I didn't want to load the record but that just makes it think that they were
of the type I requested so I got extra matches.

2. You have to map all objects stored in the table

At the moment, I don't care about a lot of the data in the table I'm using
but OJB insists on loading everything and then starts failing when it finds
a type I haven't mapped in the repository yet. It would be really nice if
these objects could be silently ignored.

3. The SQL query could be improved

If you load all objects of a derived type then OJB actually retrieves every
record in the table. It seems to me that since the repository knows the
inheritance graph for all these classes it could construct a where clause to
reduce the number of records loaded.

Just in case that's not clear, here's a contrived example. Given a class
hirearchy of

    Fruit
        Orange
            SevilleOrange
            Satsuma   (yeah, I know it's not an orange - I said it was
contrived)
        Banana

When you ask for all Orange objects the where clause could be "where
ojbConcreteClass in ('Orange', 'SevilleOrange', 'Satsuma')".

In fact when you ask for all Fruit objects it could build a where clause for
all types in the hierarchy and any unmapped types would not be retrieved.


It looks like the implementation of 3 would actually solve 1 and 2.

Thomas, can you see any problems with this - I have very limited knowledge
of OJB's implementation at the moment.

Cheers, Matt

---
Matt Goodall
Analyst Programmer, Strategic Systems Solutions
e: mat...@ss..., t: +44 113 3892645