Explain infront of a
select query shows you how the query will be executed. This way you to see if the query uses an index or if you could optimize your query by adding an index.
explain select * from user join data on user.test = data.fk_user;
id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE user index test test 5 (null) 1 Using where; Using index 1 SIMPLE data ref fk_user fk_user 5 user.test 1 (null)
type you see if an index was used. In the column
possible_keys you see if the execution plan can choose from different indexes of if none exists.
key tells you the acutal used index.
key_len shows you the size in bytes for one index item. The lower this value is the more index items fit into the same memory size an they can be faster processed.
rows shows you the expected number of rows the query needs to scan, the lower the better.