Indexes on tables

Hello,
it seems that the database on vicidial is not indexed by default and I found out that there are quite a lot of queries, which could (probably) profit from adding some indexed. First of all, is there any reason why not to add these indexes? And then, did someone already tried to analyze this and has a list of indexes to be made?
Mysql variables, which are too high on our server :
Handler_read_rnd
Handler_read_rnd_next
Select_full_join
Thanks, Lukas
it seems that the database on vicidial is not indexed by default and I found out that there are quite a lot of queries, which could (probably) profit from adding some indexed. First of all, is there any reason why not to add these indexes? And then, did someone already tried to analyze this and has a list of indexes to be made?
Mysql variables, which are too high on our server :
Handler_read_rnd
Handler_read_rnd_next
Select_full_join
Thanks, Lukas