Is normal MySql index required when we have Composite index

Question

We have MySql table having 25 million rows

Following are the columns in table

c_id,c_name,s_id,l_type,l_time,message,domain

among above

Answer 1

Any prefix of a composite index will be used as an index by itself as well. So if you have a composite index on (domain, log_time, log_type), it's equivalent to having indexes on domain and (domain, log_time). There's no need to have these indexes separately as well, they will be redundant and waste space.

So you can safely remove the index on domain when you add this composite index.