Re: [HACKERS] Foreign Join pushdowns not working properly for outerjoins

On 2017/03/06 21:22, Ashutosh Bapat wrote:
> On Mon, Mar 6, 2017 at 1:29 PM, David Rowley
> <david.rowley@2ndquadrant.com> wrote:
>> On 6 March 2017 at 18:51, Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote:
>>> On 2017/03/06 11:05, David Rowley wrote:
>> I looked over yours and was surprised to see:
>>
>> + /* is_foreign_expr might need server and shippable-extensions info. */
>> + fpinfo->server = fpinfo_o->server;
>> + fpinfo->shippable_extensions = fpinfo_o->shippable_extensions;
>>
>> That appears to do nothing for the other server options. For example
>> use_remote_estimate, which is used in estimate_path_cost_size(). As of
>> now, that's also broken. My patch fixes that problem too, yours
>> appears not to.

Thanks for the comments!  Actually, those options including 
use_remote_estimate are set later in foreign_join_ok, so 
estimate_path_cost_size would work well, for example.

>> Even if you expanded the above code to process all server options, if
>> someone comes along later and adds a new option, my code will pick it
>> up, yours could very easily be forgotten about and be the cause of yet
>> more bugs.

I agree with you on that point.

>> It seems like a much better idea to keep the server option processing
>> in one location, which is what I did.

Seems like a better idea.

> I agree with this. However
> 1. apply_server_options() is parsing the options strings again and
> again, which seems wastage of CPU cycles. It should probably pick up
> the options from one of the joining relations. Also, the patch calls
> GetForeignServer(), which is not needed; in foreign_join_ok(), it will
> copy it from the outer relation anyway.
> 2. Some server options like use_remote_estimate and fetch_size are
> overridden by corresponding table level options. For a join relation
> the values of these options are derived by some combination of
> table-level options.
>
> I think we should write a separate function
> apply_fdw_options_for_joinrel() and pass the fpinfos of joinrel, outer
> and inner relation. The function will copy the values of server level
> options and derive values for table level options. We would add a note
> there to keep this function in sync with apply_*_options(). I don't
> think there's any better way to keep the options in sync for base
> relations and join relations.
>
> Here's the patch attached.

I looked over the patch quickly.  I think it's a better idea that to 
gather various option processing in foreign_join_ok (or 
foreign_grouping_ok) in one place.  However, I'm wondering we really 
need to introduce apply_table_options and apply_server_options.  ISTM 
that those option processing in postgresGetForeignRelSize is gathered in 
one place well already.

Best regards,
Etsuro Fujita