The Jørgen's point of view, taken from http://jorgenloland.blogspot.in/2012/04/improvements-for-many-table-joins-in.html, states that:
"MySQL uses greedy search algorithm to to find the best order to join tables. When you join just a few tables, there's no problem calculating the cost of all join order combinations and then pick the best plan. However, since there are (#tables)! possible combinations, the cost of calculating them all soon becomes too high: for five tables, e.g., there are 120 combinations which is no problem to compute. For 10 tables there are 3.6 million combinations and for 15 tables there are 1307 billion. For this reason, MySQL makes a trade off: use heuristics to only explore promising plans. This ...
