The document discusses using a radix tree data structure in PL/SQL to efficiently look up international calling codes. It begins by describing the problem of slow lookups of calling codes using SQL queries. It then introduces the radix tree as a solution, defining it as a space-optimized trie that can represent the prefix data. The document outlines implementing a radix tree in PL/SQL, including bulk loading, validation, and lookup methods. It finds that lookups using the radix tree are much faster than the SQL queries, performing in 0.00011 seconds on average for a batch of calls.