On Sat, Apr 30, 2016 at 12:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Fix planner crash from pfree'ing a partial path that a GatherPath uses.
>
> We mustn't run generate_gather_paths() during add_paths_to_joinrel(),
> because that function can be invoked multiple times for the same target
> joinrel. Not only is it wasteful to build GatherPaths repeatedly, but
> a later add_partial_path() could delete the partial path that a previously
> created GatherPath depends on. Instead establish the convention that we
> do generate_gather_paths() for a rel only just before set_cheapest().
>
> The code was accidentally not broken for baserels, because as of today there
> never is more than one partial path for a baserel. But that assumption
> obviously has a pretty short half-life, so move the generate_gather_paths()
> calls for those cases as well.
>
> Also add some generic comments explaining how and why this all works.
>
> Per fuzz testing by Andreas Seltenreich.
>
> Report: <871t5pgwdt.fsf@credativ.de>
Thanks!
The preposition at the end of this sentence broke my internal English parser:
+ * This must not be called until after we're done creating all partial paths
+ * for the specified relation. (Otherwise, add_partial_path might delete a
+ * path that some GatherPath has a reference to.)
I eventually figured out what you were saying there, and it does make sense.
--
Robert Haas
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