Basically Nested Set is optimized for traversing trees, as this can be done with minimal queries, however updating the tree can be costly as it will affect all rows within the table. For more information, read here: [http://www.sitepoint.com/article/hierarchical-data-database/2 http://www.sitepoint.com/article/hierarchical-data-database/2], [http://dev.mysql.com/tech-resources/articles/hierarchical-data.html http://dev.mysql.com/tech-resources/articles/hierarchical-data.html]
