B+tree index - Simple SQL implementation

Implementation of B + Tree in declarative programming language environment (DBMS): After searching for existing implementations should transfer customize one of these, or start your own one, a DBMS of your choice. The language implementation will obviously be the SQL and / or PL / SQL - / - T-SQL - / - PL pgSQL. The tree will be saved in database (tables) and not to drive as usual. The implementation should support only 2 operations: (1) introducing a new element to the index(tree) and (2) search in 2 versions, (a) identity and (b) range. Attention: I want the implementation to be very simple and understandable. Furthermore i want to be explained with comments into the code.