One weakness of Adjacency List is that it’s expensive to retrieve ancestors of a given node in the tree. In Path Enumeration, this is solved by storing the string of ancestors as an attribute of each node.
You can see a form of Path Enumeration in directory hierarchies. A UNIX path like /usr/local/lib/ is a Path Enumeration of the filesystem, where usr is the parent of local, which in turn is the parent of lib.
In the Comments table, instead of the parent_id column, define a column called path as a long VARCHAR. The string stored in this column is the sequence of ancestors of the current row in order from the top of the tree down, just like a UNIX path. You can even choose / as a separator character. (查看原文)
In the television series Star Trek,1 “tribbles” are small furry animals
kept as pets. Tribbles are very appealing at first, but soon they reveal
their tendency to reproduce out of control, and managing the overpopulation
of tribbles becomes a serious problem. (查看原文)
