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In the initial cut at LATERAL, I kept the rule that cheapest_total_path was always unparameterized, which meant it had to be NULL if the relation has no unparameterized paths. It turns out to work much more nicely if we always have *some* path nominated as cheapest-total for each relation. In particular, let's still say it's the cheapest unparameterized path if there is one; if not, take the cheapest-total-cost path among those of the minimum available parameterization. (The first rule is actually a special case of the second.) This allows reversion of some temporary lobotomizations I'd put in place. In particular, the planner can now consider hash and merge joins for joins below a parameter-supplying nestloop, even if there aren't any unparameterized paths available. This should bring planning of LATERAL-containing queries to the same level as queries not using that feature. Along the way, simplify management of parameterized paths in add_path() and friends. In the original coding for parameterized paths in 9.2, I tried to minimize the logic changes in add_path(), so it just treated parameterization as yet another dimension of comparison for paths. We later made it ignore pathkeys (sort ordering) of parameterized paths, on the grounds that ordering isn't a useful property for the path on the inside of a nestloop, so we might as well get rid of useless parameterized paths as quickly as possible. But we didn't take that reasoning as far as we should have. Startup cost isn't a useful property inside a nestloop either, so add_path() ought to discount startup cost of parameterized paths as well. Having done that, the secondary sorting I'd implemented (in add_parameterized_path) is no longer needed --- any parameterized path that survives add_path() at all is worth considering at higher levels. So this should be a bit faster as well as simpler. |
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