Re: [HACKERS] Foreign Data Wrapper

[Big Mike]

Inspecting the multicorn source was helpful.

For the purposes of anyone searching the mailing lists, I needed to access
the quals natively. The code that helped me figure out how to do this was
in the native odbc fdw here:
https://github.com/ZhengYang/odbc_fdw/blob/master/odbc_fdw.c

particularly the odbcBeginForeignScan() and odbcIterateForeignScan(), and
odbcGetQual() functions.


Thanks

On Fri, Nov 13, 2015 at 1:24 PM, Corey Huinker 
wrote:

> On Fri, Nov 13, 2015 at 1:46 PM, Big Mike  wrote:
>
>> Writing a Foreign Data Wrapper and interested in isolating the WHERE
>> clause to speed up the access of an indexed file on my filesystem. I'm
>> attempting to understand the inner workings of how the data is retrieved so
>> I'm writing code to just handle one case at the moment: WHERE clause on a
>> single column in the foreign 'table'.
>>
>> SELECT * FROM t WHERE testval = 1
>>
>> I have this code so far, an implementation of the IterateForeignScan
>> interface.
>>
>> static TupleTableSlot *
>> bIterateForeignScan(ForeignScanState *node) {
>> ...
>> RestrictInfo *rinfo = (RestrictInfo *)node->ss.ps.qual;
>> ...
>> }
>>
>> yet am not familiar with what I need to do to pick apart RestrictInfo in
>> order to gather 'testvar', '=', and '1' separately so I can interpret and
>> pass those through to my file parser.
>>
>> Am I going about this the correct way or is there another path I should
>> follow?
>>
>
>
> I would look at http://multicorn.org/ which gives you a working python
> framework. You subclass their ForeignDataWrapper class, override the
> __init__() and execute() functions, and that's about it. The execute()
> function has a list called quals that would set you up for the filtering
> you want to do.
>
> I would get the foreign data wrapper fully debugged this way before
> attempting to refactor in C. And when you do finally re-code, you can see
> what multicorn itself did to implement your filters.
>
>
>
>
>
>
>
>

[HACKERS] Foreign Data Wrapper

[Big Mike]

Writing a Foreign Data Wrapper and interested in isolating the WHERE clause
to speed up the access of an indexed file on my filesystem. I'm attempting
to understand the inner workings of how the data is retrieved so I'm
writing code to just handle one case at the moment: WHERE clause on a
single column in the foreign 'table'.

SELECT * FROM t WHERE testval = 1

I have this code so far, an implementation of the IterateForeignScan
interface.

static TupleTableSlot *
bIterateForeignScan(ForeignScanState *node) {
...
RestrictInfo *rinfo = (RestrictInfo *)node->ss.ps.qual;
...
}

yet am not familiar with what I need to do to pick apart RestrictInfo in
order to gather 'testvar', '=', and '1' separately so I can interpret and
pass those through to my file parser.

Am I going about this the correct way or is there another path I should
follow?