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POManaging hierarchies in SQL: MPTT/nested sets vs adjacency lists vs storing paths
 

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  1. POManaging hierarchies in SQL: MPTT/nested sets vs adjacency lists vs storing paths
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    <p>For a while now I've been wrestling with how best to handle hierarchies in SQL. Frustrated by the limitations of adjacency lists and the complexity of MPTT/nested sets, I began thinking about simply storing key paths instead, as a simple <code>node_key/node_key/...</code> string. I decided to compile the pros and cons of the three techniques:</p> <h3>Number of calls required to create/delete/move a node:</h3> <ul> <li>Adjacency = 1</li> <li>MPTT = 3</li> <li>Path = 1 (Replace old node path with new node path across all nodes that contain that path)</li> </ul> <h3>Number of calls required to get a tree:</h3> <ul> <li>Adjacency = [number of sub-levels]</li> <li>MPTT = 1</li> <li>Path = 1</li> </ul> <h3>Number of calls required to get path to a node / ancestry:</h3> <ul> <li>Adjacency = [number of super-levels]</li> <li>MPTT = 1</li> <li>Path = 0</li> </ul> <h3>Number of calls required to get number of subnodes:</h3> <ul> <li>Adjacency = [number of sub-levels] </li> <li>MPTT = 0 (Can be calculated from right/left values) </li> <li>Path = 1</li> </ul> <h3>Number of calls required to get depth of node:</h3> <ul> <li>Adjacency = [number of super-levels]</li> <li>MPTT = 1</li> <li>Path = 0</li> </ul> <h3>DB fields required:</h3> <ul> <li>Adjacency = 1 (parent)</li> <li>MPTT = 3 (parent,right,left)</li> <li>Path = 1 (path)</li> </ul> <h1>Conclusion</h1> <p>The stored path technique uses the same or less calls than the other techniques in every use case except one. By this analysis, storing paths is a clear winner. Not to mention, it's a lot simpler to implement, human readable, etc. </p> <p>So the question is, shouldn't stored paths be considered a stronger technique than MPTT? Why are stored paths not a more commonly used technique, and why would you not use them over MPTT in a given instance? </p> <p>Also, if you think this analysis is incomplete please let me know.</p> <h3>UPDATE:</h3> <p>Here are at least 2 things MPTT can do out of the box that a stored path solution won't:</p> <ol> <li>Allows calculation of subnode count for each node without any additional queries (mentioned above).</li> <li>Imposes an order on nodes at a given level. The other solutions are unordered.</li> </ol>
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    1. COIt is actually not true that you need [number of sub-levels] of calls in an adjacency model. You can do something like : `SELECT * FROM adjacency a /* for($number_of_levels_required.. $prevname = $level > 1 ? 'a'+$level-1 : 'a'; */ INNER JOIN adjacency a/*$level*/ ON a/*$level*/.parent_id = /*$prevname*/.id /* endfor*/`
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