On Wed, Sep 29, 2010 at 11:01 AM, Robert Haas <robertmh...@gmail.com> wrote: > Makes sense to me. It seems that there are actually two halves to > this problem: getting the child EMs to be generated in the first > place, and then getting them to be examined at the appropriate time.
So I tried out the logic described in this email and, with a few modifications, it seemed to work. Updated patch attached, any review appreciated. There are still a bunch of other things that need to be fixed here, but I think this is OK as far as this particular issue is concerned. I fixed a few other things: - create_append_plan must call create_plan_recurse rather than create_plan. This appears to be an error introduced by rebasing; the previous version of the patch seg faults when attempting to execute a plan wherein an inner index scan has been pushed down through an append node - remove the hack to short-circuit the append node altogether if there's only one child - some miscellaneous code cleanup (more is needed) > 3. TGL: "Speaking of sorting, it's not entirely clear to me how the > patch ensures that all the child plans produce the necessary sort keys > as output columns, and especially not how it ensures that they all get > produced in the *same* output columns. This might accidentally manage > to work because of the "throwaway" call to make_sort_from_pathkeys(), > but at the very least that's a misleading comment." I'm not sure what > needs to be done about this; I'm going to look at this further. I spent some time looking at this complaint, and I'm still not sure what needs to be done about it. Experimentation reveals that the neither the throwaway call to make_sort_from_pathkeys() nor any other call to that function is creating the relevant target list entries. It appears that they're getting created when set_append_rel_pathlist() calls adjust_appendrel_attrs(). I'm not sure whether that's sufficient or not. make_sort_from_pathkeys() could add any missing entries later, but I'm not too sure the order would match if that happened. Another awkwardness of this patch is that it makes create_append_path() and consequently set_dummy_rel_pathlist() take an additional "root" argument. While there's nothing terribly unreasonable about this on its face, it's only necessary so that create_append_path() can call cost_sort(), which takes "root" but doesn't actually use it. I'm not sure whether it's better to leave this as-is or to remove the root argument from cost_sort(). -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company
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