[sqlite] why is searching for a range of rowids in an FTS table a slow operation?

[skywind mailing lists]

Hi,

according to the documentation this is a slow query for FTS tables:

SELECT * FROM mail WHERE rowid BETWEEN 15 AND 20;

while 

SELECT * FROM mail WHERE rowid=15;

is fast.
As far as I know both queries are fast on normal tables. Where is the 
difference / what is the reason for the performance difference?

Regards,
Hartwig