Re: [HACKERS] Parallel Hash take II

On Sat, Sep 2, 2017 at 10:45 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Sep 1, 2017 at 6:32 PM, Thomas Munro
> <thomas.munro@enterprisedb.com> wrote:
>> On Sat, Sep 2, 2017 at 5:13 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> On Thu, Aug 31, 2017 at 8:53 AM, Thomas Munro
>>> <thomas.munro@enterprisedb.com> wrote:
>>>> Check out ExecReScanGather(): it shuts down and waits for all workers
>>>> to complete, which makes the assumptions in ExecReScanHashJoin() true.
>>>> If a node below Gather but above Hash Join could initiate a rescan
>>>> then the assumptions would not hold.  I am not sure what it would mean
>>>> though and we don't generate any such plans today to my knowledge.  It
>>>> doesn't seem to make sense for the inner side of Nested Loop to be
>>>> partial.  Have I missed something here?
>>>
>>> I bet this could happen, although recent commits have demonstrated
>>> that my knowledge of how PostgreSQL handles rescans is less than
>>> compendious.  Suppose there's a Nested Loop below the Gather and above
>>> the Hash Join, implementing a join condition that can't give rise to a
>>> parameterized path, like a.x + b.x = 0.
>>
>> Hmm.  I still don't see how that could produce a rescan of a partial
>> path without an intervening Gather, and I would really like to get to
>> the bottom of this.
>
> I'm thinking about something like this:
>
> Gather
> -> Nested Loop
>   -> Parallel Seq Scan
>   -> Hash Join
>     -> Seq Scan
>     -> Parallel Hash
>       -> Parallel Seq Scan
>
> The hash join has to be rescanned for every iteration of the nested loop.

I think you mean:
Gather-> Nested Loop  -> Parallel Seq Scan  -> Parallel Hash Join    -> Parallel Seq Scan    -> Parallel Hash      ->
ParallelSeq Scan
 

... but we can't make plans like that and they would produce nonsense
output.  The Nested Loop's inner plan is partial, but
consider_parallel_nestloop only makes plans with parallel-safe but
non-partial ("complete") inner paths.

/** consider_parallel_nestloop*        Try to build partial paths for a joinrel by joining a
partial path for the*        outer relation to a complete path for the inner relation.*

-- 
Thomas Munro
http://www.enterprisedb.com