1. A database index for indexing one or more text documents ( 10 ) in a database, S the text documents ( 10 ) comprising one or more hierarchical nodes ( 10 a , 10 b , 10 c ) each comprising one or more words ( 20 a ,. .. , 20 f ), the database index comprising at least one entry ( 50 ), each entry ( 50 ) comprising: a. a key ( 51 ) comprising a subset ( 510 ) of words occurring in one of the hierarchical nodes ( 10 a , 10 b , 10 c ) of the text documents ( 10 ) and the name ( 511 ) of the respective hierarchical node ( 10 a , 10 b , 10 c ); and b. a value ( 52 ) comprising one or more references ( 520 ) to the text documents ( 10 ) in which the subset ( 510 ) occurs.