This replaces the former modified preorder tree traversal (MPTT) with the
fields `parent_left`/`parent_right`. Each record is associated to a string
`parent_path`, that represents the path from its root node to itself. The path
is made of the node ids suffixed with a slash:
a node | id | parent_path
/ \ a | 42 | 42/
... b b | 63 | 42/63/
/ \ c | 84 | 42/63/84/
c d d | 85 | 42/63/85/
This field provides an efficient implementation for parent_of/child_of queries:
the nodes in the subtree of record are the ones where `parent_path` starts with
the `parent_path` of record. It is also more efficient to maintain than the
MPTT fields, and less sensitive to concurrent updates, because the value of
`parent_path` does not depend on sibling nodes.
Raphael Collet
e724858d50