Programming the Perl DBI

Updating Data

Updating data within a flat-file database is where things begin to get a little more tricky. When querying records from the database, we simply scanned sequentially through the database until we found the correct record. Similarly, when inserting data, we simply attached the new data without really knowing what was already stored within the database.

The main problem with updating data is that we need to be able to read in data from the data file, temporarily mess about with it, and write the database back out to the file without losing any records.

One approach is to slurp the entire database into memory, make any updates to the in-memory copy, and dump it all back out again. A second approach is to read the database in record by record, make any alterations to each individual record, and write the record immediately back out to a temporary file. Once all the records have been processed, the temporary file can replace the original data file. Both techniques are viable, but we prefer the latter for performance reasons. Slurping entire large databases into memory can be very resource-hungry.

The following short program implements the latter of these strategies to update the map reference in the database of delimited records:

#!/usr/bin/perl -w # # ch02/updatemegadata/updatemegadata: Updates the given megalith data file # for a given site. Uses colon-separated # data and updates the map reference field. # ### Check the user has supplied an argument to scan for ### 1) The ...