On 2019/03/28 14:03, Tom Lane wrote:
> Amit Langote <Langote_Amit_f8@lab.ntt.co.jp> writes:
>> On 2019/03/27 23:57, Tom Lane wrote:
>>> Yeah, there's something to be said for having plancat.c open each table
>>> *and store the Relation pointer in the RelOptInfo*, and then close that
>>> again at the end of planning rather than immediately. If we can't avoid
>>> these retail table_opens without a great deal of pain, that's the
>>> direction I'd tend to go in. However it would add some overhead, in
>>> the form of a need to traverse the RelOptInfo array an additional time.
>
>> Just to be sure, do you mean we should do that now or later (David said
>> "in the long term")?
>
> It's probably not high priority, though I wonder how much time is being
> eaten by the repeated table_opens.
It took me a couple of hours to confirm this, but it seems significantly
less than it takes to go over the simple_rel_array one more time to close
the relations. The scan of simple_rel_array to close the relations needs
to be done once per query_planner() invocation, whereas I'd hoped this
could be done, say, in add_rtes_to_flat_rtable() that has to iterate over
the range table anyway. That's because we call query_planner multiple
times on the same query in a few cases, viz. build_minmax_path(),
inheritance_planner().
Within query_planner(), table is opened in expand_inherited_rtentry(),
get_relation_constraints(), get_relation_data_width(), and
build_physical_tlist(), of which the first two are more common. So, we
end up doing table_open() 3 times on average for any given relation, just
inside query_planner().
Thanks,
Amit