How do I choose what fields to index and which ones not to? Is the index useful for when I set a condition in the database query for a quick look up of the rows? I guess this is an SQL question but I really want to know what indexes are used for Drupal entities.
What are all these indexes in the node table for?
'indexes' => array(
'node_changed' => array('changed'),
'node_created' => array('created'),
'node_moderate' => array('moderate'),
'node_frontpage' => array('promote', 'status', 'sticky', 'created'),
'node_status_type' => array('status', 'type', 'nid'),
'node_title_type' => array('title', array('type', 4)),
'node_type' => array(array('type', 4)),
'uid' => array('uid'),
'tnid' => array('tnid'),
'translate' => array('translate'),
),