Add an index on depth

Problem/Motivation

This subquery is generated when you use a relationship to the Root node in a view. It has performance issues resolved by adding an index on depth. EXPLAIN.

SELECT eh_child.id AS eh_child_id, eh_child.revision_id AS eh_child_revision_id, eh_parent.id AS nid
FROM
nested_set_field_parent_node eh_child
INNER JOIN nested_set_field_parent_node eh_parent ON (eh_child.left_pos BETWEEN eh_parent.left_pos AND eh_parent.right_pos) AND eh_parent.depth = 0

Steps to reproduce

* Create a big tree of nodes
* Create a view on the nodes, and add a relationship to Root/Top node.
* Analyse the performance of the resulting query

Proposed resolution

Add an index on depth.