Taxonomy Depth

The Taxonomy Depth module enables dramatically more efficient taxonomy "depth" searches than the core taxonomy module.

Drupal vocabularies can be hierarchical; terms can have parents. A normal taxonomy search is "retrieve all nodes with term T." A depth 1 taxonomy search is "retrieve all nodes with term T or any of term T's children." A depth N search is "retrieve all nodes with term T or any of its descendents up to N levels deep."

When Taxonomy module performs a depth D search for a term T, it retrieves every term in T's vocabulary, scans the results in PHP to compute the descendant graph dynamically, extracts the descendants of T of depth D or less, and executes "SELECT FROM node INNER JOIN term_node WHERE tid IN (all terms in the descendant list)".

When Views module performs a depth D search for a term T, it joins to the term_hierarchy and term_data tables once for *every* depth level, plus the initial join to term_node, for a total of 2*D+1 joins. If you define a page view with three separate Term ID arguments, each of which does a depth 3 search, you end up with 21 joins in addition to whatever else the view requires

When Taxonomy Depth module performs a depth D search for a term T, it joins to the term_node and term_tables once each. The page view described above with three depth three searches then only requires 6 joins instead of 21.

This module is pre-release.

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