If the concept of function-based indexing is new to you, you might be tempted to just index everything, but this is in fact the very last thing you should do. The reason is that every index causes ongoing maintenance. Function-based indexes are particularly troublesome because they make it very easy to create redundant indexes.
The case-insensitive search from above could be implemented with the
LOWER function as well:
SELECT first_name, last_name, phone_number FROM employees WHERE LOWER(last_name) = LOWER('winand');
A single index cannot support both methods of ignoring the case. We could, of course, create a second index on
LOWER(last_name) for this query, but that would mean the database has to maintain two indexes for each insert, update, and delete statement (see also Chapter 8, “Modifying Data”). To make one index suffice, you should consistently use the same function throughout your application.
Unify the access path so that one index can be used by several queries.
Sometimes ORM tools use
LOWER without the developer’s knowledge. Hibernate, for example, injects an implicit
LOWER for case-insensitive searches.
Always aim to index the original data as that is often the most useful information you can put into an index.