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Re: foreign data wrappers

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Guillaume Lelarge <guillaume(at)lelarge(dot)info>
Cc: Zheng Yang <zhengyang4k(at)gmail(dot)com>, Shiv <rama(dot)theone(at)gmail(dot)com>, Selena Deckelmann <selena(at)chesnok(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, pgsql-students <pgsql-students(at)postgresql(dot)org>
Subject: Re: foreign data wrappers
Date: 2011-03-29 16:32:13
Message-ID: 4D92098D.6040409@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-students

On 03/29/2011 11:48 AM, Guillaume Lelarge wrote:
> Le 29/03/2011 13:28, Zheng Yang a écrit :
>> Hi guys,
>>
>>>> I've briefly gone through the slides. Regarding the 6 callbacks, is that correct to say that a full table scan will always be performed irregardless of the sql statement,
>>>> the FDW is blind to the sql query performed, right?
>>> Yes, fairly much. If the feed is large you need some way to pass a limit to the foreign side, possibly via table options. I'm fairly sure you won't be able to get it via the SELECT statement.
>>>
>>
>> Regarding the previous flickr example, I'm wondering how this 'free text search' function can be done if the FDW is blind to the SELECT statement.
>>
>> For instance, the following query is to retrieve a photo relevant to 'panda':
>>
>> 	SELECT photo FROM flickr_table WHERE search LIKE '%panda%';
>>
>> In this case, the FDW can only open a connection to flickr web service and return the next 'row' .
>> The problem is that there are a huge number of photos in flickr server and retrieving them sequentially is not realistic.
>> Any ideas on how this can be done?
>>
> It probably means that flickr is not a good example of a nice fdw.


Neither of you are being very creative. As I mentioned above, you need 
to embed this sort of stuff in table options.

so you would have something like:

    create foreign table panda_flickr (photo bytea, ...)
         server flickr_server
         options (searchterm 'panda', maxrows '50');
    select photo from panda_flickr;


cheers

andrew

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