Translated
Translated
Updated according to feedback
Updated according to feedback
Merge pull request #239 from jcarlosgarcia/when-index-used

Completed translation of "12-When is an index used" parts a and b. Resolves #234
Merge pull request #243 from jcarlosgarcia/how-large-index

Completed translation of "13-How large is your index" parts a and b. Resolves #240