When comparing indexation results between MySQL, PostgreSQL and SQLite, MySQL records more symbol usages in table lxr_usages. After diff'ing the tables, it appears the extra usages hit on similar looking symbols (with a different case). MySQL manual says: "By default, string comparisons are not case sensitive and use the current character set." This may be correct for natural languages but definitely wrong for computer science and case-sensitive languages.
The easiest way to solve that issue is to change the type of the columns in the table description to BINARY. Thus, we keep method common factoring in the Index class.