Re: [COMMITTERS] pgsql: Fix planner crash from pfree'ing a partial path that a GatherPat

On Sat, Apr 30, 2016 at 12:29 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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(dot)fsf(at)credativ(dot)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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company