understanding mysql indexes

I have read a couple of articles but im still having trouble understanding this. I have seen the difference of performance using EXPLAIN. What i dont understand is would i need to do something when the tables are beeing altered? Say i开发者_如何学Go delete some rows or add some rows, do i need to create an index for every time or do i need to alter an index or something like that or does mysql fix this itself?

I just did a :

CREATE UNIQUE INDEX rel_index
ON friend_relations (from_id, to_id);

do i need to do some spesific queries when the tables are altered? im coming here becouse i dont really understand how mysql does this.. even if i have checked around.

best of regards, alexander elias nilsen

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No, you don't have to adjust the index every time you change the data. When you first create the index, MySQL will populate the index data for pre-existing rows. Then, when you add, remove, or modify rows, MySQL will automatically update the index data.