Programming the Perl DBI

Querying limitations of DBM files and hashtables

Even with the functionality of being able to insert complex data into the Berkeley DB file (albeit in a slightly roundabout way), there is still a fundamental limitation of this database software: you can retrieve values via only one key. That is, if you wanted to search our megalithic database, the name, not the map reference or the location, must be used as the search term.

This might be a pretty big problem, given that you might wish to issue a query such as, ``tell me about all the sites in Wiltshire,'' without specifying an exact name. In this case, every record would be tested to see if any fit the bill. This would use a sequential search instead of the indexed access you have when querying against the key.

A solution to this problem is to create secondary referential hashes that have key values for the different fields you might wish to query on. The value stored for each key is actually a reference to the original hash and not to a separate value. This allows you to update the value in the original hash, and the new value is automatically mirrored within the reference hashes. The following snippet shows some code that could be used to create and dump out a referential hash keyed on the location of a megalithic site:

### Build a referential hash based on the location of each monument $locationDatabase{'Wiltshire'} = \$database{'Avebury'}; $locationDatabase{'Western Isles'} = \$database{'Callanish I'}; $locationDatabase{'Fife'} ...