Understanding Lucene leading wildcard performance

Question

Lucene does not by default allow leading wildcards in search terms, but this can be enabled with:

QueryParser#setAllowLeadingWildcard(true)

Answer 1

If I understand it correctly, a part of the index is the term dictionary, which is a sorted list of all the indexed terms. When searching with no wildcards or a trailing wildcard, Lucene can take advantage of the fact that many terms have common prefixes. On the other hand, searching with a leading wildcard scans the entire term dictionary. This is not optimal, but the term dictionary tends to be tiny compared to other parts of the index, such as frequency and position data, so a full scan of the term dictionary is usually not big problem by itself.