Michael Lewis <mlewis@xxxxxxxxxxx> writes: >> When you join to a view, the view sticks together, as if they were all in >> parentheses. But when you substitute the text of a view into another >> query, then they are all on the same level and can be parsed differently. >> >> Consider the difference between "1+1 * 3", and "(1+1) * 3" > I thought from_collapse_limit being high enough meant that it will get > re-written and inlined into the same level. To extend your metaphor, that > it would be 1 * 3 + 1 * 3. The point is that the semantics are actually different --- in Jeff's example, the answer is 4 vs. 6, and in the OP's query, the joins have different scopes. from_collapse_limit has to do with whether the planner can rewrite the query into a different form, but it's not allowed to change the semantics by doing so. In some cases you can re-order joins without changing the semantics, just as arithmetic has associative and commutative laws. But you can't always re-order outer joins like that. I didn't dig into the details of the OP's query too much, but I believe that the two forms of his join tree are semantically different, resulting in different runtimes. regards, tom lane
