join/from_collapse_limit and geqo_threshold default values

[Vlad Arkhipov <arhipov@xxxxxxxxxxxx>]

I wonder why join_collapse_limit default values is set to 8 but geqo_threshold is 12. Optimizer doesn't change the order of JOIN's of queries that contains from 8 to 11 tables. Why it's 'wise' decision as documentation says?

from_collapse_limit (integer)

The planner will merge sub-queries into upper queries if the resulting FROM list would have no more than this many items. Smaller values reduce planning time but might yield inferior query plans. The default is eight. It is usually wise to keep this less than geqo_threshold. For more information see Section 14.3.