Amount of index influence on performance?

Question

I have a relatively large database.

And the consultation in it varies a lot and I have some doubts regarding performance:

1- Create multiple indexes, taking into account the

Where (E.condicao = '1') AND (E.situacao != 'disabled')...

Does it influence performance? (Beyond disk space?)

2 - How it defines which index to use?

3 - Is there any weighting with regard to using multiple indices?

Using Mysql 5.5.

Answer 1

1 - Create multiple indexes, taking into account the

Where (E.condicao = '1') AND (E.situacao != 'desativado')...

I put in the Github for future reference.

Does it influence performance? (Beyond disk space?)

I think it was answered in What are the advantages and disadvantages of using indexes in databases?.

2 - How it defines which index to use?

Using relational algebra and set theory comparing with concrete and statistical data from the database to obtain heuristics which determine that one of the existing indices will produce result more efficiently, or that an index to be created only for the operation may offset its cost.

In this example he will look for an index that is composed by the column condicao and if possible followed by the column situacao, which by the way seems an inefficient column.

3 - Is there any weighting with regard to using multiple indices?

You can see this in Unnecessary bank indices are a problem?.