Hierarchical data structures (in a single table)

Question

Hi,
If I have a hierarchy of objects stored in a table -
ORG_UNIT
ID
PARENT_ID
NAME
And the JDO mapping for an OrgUnit contains a parent OrgUnit and a
Collection of children.
Is there an efficient way of pulling them out of the database.
It is currently loading each individual parent's kids.
This is going to be pretty slow if there are say 500 OrgUnits in the
database.
If it would be better to pull them all out and build the hierarchy up in
code (as it was being done in straight JDBC). How can I efficiently obtain
the parent or children without doing exactly the same?
Thanks,
Simon

Answer 1

Simon,
There will be no db access for every child - you will read all child records
for a particular parent at once when you try to access its child collection.
Granted that for terminal leaves you will get an db access to load an empty
collection so effectively you will get a db access per node. If your goal is
always to load and traverse entire tree it will be expensive. But the
beauty of hierarchical structures is that while they could be huge millions
of nodes you do not need to load it all to navigate - just the path you
need. This is where lazy loading excels so overall on large trees you will
be much better of not loading whole thing at once. However if you still want
to do it nothing prevents you from not having persistent collection of child
records in OrgUnit class at all but only a reference to a parent, load
entire table using query and then build tree in memory yourself as you
iterate over the query resultset. You can probably even do it in a single
iteration over the resultset. I would never do it myself though . In my
opinion it defeats ease of use and cleanness of your object model.
Alex
"Simon Horne" <[email protected]> wrote in message
news:ag1p9p$9si$[email protected]..
Hi,
If I have a hierarchy of objects stored in a table -
ORG_UNIT
ID
PARENT_ID
NAME
And the JDO mapping for an OrgUnit contains a parent OrgUnit and a
Collection of children.
Is there an efficient way of pulling them out of the database.
It is currently loading each individual parent's kids.
This is going to be pretty slow if there are say 500 OrgUnits in the
database.
If it would be better to pull them all out and build the hierarchy up in
code (as it was being done in straight JDBC). How can I efficiently obtain
the parent or children without doing exactly the same?
Thanks,
Simon