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

On Wed, Jul 19, 2017 at 7:45 PM, Thomas Munro
<thomas.munro@enterprisedb.com> wrote:
> On Thu, Jul 20, 2017 at 7:00 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> 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.
>
> Isn't this the same as the issue reported here?
>
> https://www.postgresql.org/message-id/flat/CAEepm%3D270ze2hVxWkJw-5eKzc3AB4C9KpH3L2kih75R5pdSogg%40mail.gmail.com

Hmm, possibly.  But why would that affect the partition-wise join case only?

-- 
Robert Haas
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