Re: [HACKERS] Partition-wise join for join between (declaratively)partitioned tables

On Thu, Jul 20, 2017 at 12:30 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>
>> This suggests that partitioning is not a suitable strategy for this query,
>> but then may be partition wise should not be picked for such a case to
>> aggravate the performance issue.
>
> In the unpartitioned case, and in the partitioned case on head, the
> join order is l1-(nation-supplier)-l2-orders-l3.  In the patched case,
> the join order changes to l1-l2-supplier-orders-nation-l3.  If the
> planner used the former join order, it wouldn't be able to do a
> partition-wise join at all, so it must think that the l1-l2 join gets
> much cheaper when done partitionwise, thus justifying a change in the
> overall join order to be able to use partion-wise join.  But it
> doesn't work out.
>
> I think the problem is that the row count estimates for the child
> joins seem to be totally bogus:
>
> ->  Hash Semi Join  (cost=309300.53..491665.60 rows=1 width=12)
> (actual time=10484.422..15945.851 rows=1523493 loops=3)
>   Hash Cond: (l1.l_orderkey = l2.l_orderkey)
>   Join Filter: (l2.l_suppkey <> l1.l_suppkey)
>   Rows Removed by Join Filter: 395116
>
> That's clearly wrong.  In the un-partitioned plan, the join to l2
> produces about as many rows of output as the number of rows that were
> input (998433 vs. 962909); but here, a child join with a million rows
> as input is estimated to produce only 1 row of output.  I bet the
> problem is that the child-join's row count estimate isn't getting
> initialized at all, but then something is clamping it to 1 row instead
> of 0.
>
> So this looks like a bug in Ashutosh's patch.

The patch does not have any changes to the selectivity estimation. It
might happen that some correction in selectivity estimation for
child-joins is required, but I have not spotted any code in
selectivity estimation that differentiates explicitly between child
and parent Vars and estimates. So, I am more inclined to believe
Thomas's theory. I will try Tom's suggested approach.

I am investigating this case with the setup that Rafia provided.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company