I'm starting to use Berkeley-DB on a Linux system.

Is there a way of permanently associating a secondary index with a
primary rather than having to do it explicitly in each application?

I'm looking for the ability to add indices to a database, and
maintaining them, without have to recompile my applications. I can
define the keys in terms of integer pairs (location and length of
field in the record) so I can write a generic "callback" function to
extract the key. I could maintain a list of indices in a file and
write an "open database" function to open the primary and associate
the secondaries at run time.

Ideally, I'd like to get something similar to the system of "physical"
and "logical" files used on the AS400 (for anyone familiar with such
machines).

Any suggestions (other than "get yourself an AS400 then" ;-) ) would
be much appreciated.

--
Mike

* Mike:

No, I don't think so.

You could add this information to a configuration file, or a
designated database.

Sounds useful, but in some cases (most cases for me), column offsets
vary with column widths.

Florian Weimer wrote:

"filename" holding a list of secondary indices. The application would
be written to read through that database and open all the secondaries,
together with the primary, and automatically associate them. My
difficulty is in extracting the key fields from the data record. The
"callback" function prototype doesn't allow for suitable parameters to
be passed so, it seems, I need a separate "callback" function for each
secondary.

Yes, the method is only really suitable for fixed length records/fields.

Thanks for your reply.

--
Mike

Mike <spamdump (AT) fensoft (DOT) co.uk.invalid> writes:

The callback function gets passed a DB* (of the secondary database),
and each of these has an app_private void* field that you can use any
way you like. You could point it to a suitable const key extraction
info struct when you open the secondary database.

best regards
Patrick

Patrick Schaaf wrote:

--
Mike

* Mike:

Sometimes, people joke that the library you are looking for is called
"MySQL".

In addition to what Patrick wrote, it might be possible to use
subclassing in the C++ version of the library (I haven't checked).
You could also keep a hash table of DB pointers. My concern is that
once you use the app_private field, you lose generality (because some
application might already be using it).

Florian Weimer wrote:
Using the app_private field seems to be the answer to my problem. I
intend to write some functions "on top of" BerkeleyDB for my
application so losing generality shouldn't be a problem: I shan't
affect any existing application. If nobody used app_private, for fear
of some other application using it, there would be no point in having
it. ;-)

--
Mike