David, Please post your patch(es) and some demo of how things broke so others can improve future versions--possibly even 9.3 versions if it turns out you've discovered a bug in the implementation.
Thanks very much for your hard work and insights into this. Cheers, David. On Tue, Apr 23, 2013 at 11:08:04AM -0700, David Gudeman wrote: > In case anyone is interested, I tried it and it doesn't seem to work. > It looks like some other plan element already has the target-list > tuple baked. Now I'm trying to decide whether to give up on FDW. It's > a shame because it's such a sweet facility, but at this point, I just > don't think that it's mature enough for what I need to do. > Regards, > David Gudeman > > > On Mon, Apr 22, 2013 at 11:27 AM, David Gudeman <dave.gude...@gmail.com> > wrote: > > Re-reading my first email I thought it was a little confusing, so here > > is some clarification. In GetForeignPlan, tlist seems to be a target > > list for a basic "select *" from the foreign table. For the ith > > TargetEntry te in tlist, it seems that te->expr is a var with > > varattno=i. I was mis-remembering and calling varattno "attrno" in the > > original email. > > > > My assumption is that the plan elements that use the output of the FDW > > plan node will access columns indirectly using tlist. In other words, > > I'm assuming that if there is a reference to a column c of the foreign > > table, this column will be represented as a Var with varattno being an > > offset into tlist. So if c is column number 3, for example, you get > > its value by looking up TargetEntry number 3 in tlist and evaluate the > > expr column for that TargetEntry. So if I change the Var in the expr > > column so the varattno points to a different column in the output > > tuple, then everything will work. > > > > The two risky assumptions I'm making are 1. that it actually uses this > > indirect way of looking up columns in a foreign table and 2. that it > > actually uses the tlist that I pass in when I call make_foreignscan(). > > > > Can anyone confirm or deny these assumptions? > > > > Thanks. > > > > On Sun, Apr 21, 2013 at 6:57 PM, David Gudeman <dave.gude...@gmail.com> > > wrote: > >> > >> A few years ago I wrote a roll-your-own foreign-data-wrapper system for > >> Postgres because Postgres didn't have one at the time (some details here > >> (http://unobtainabol.blogspot.com/2013/04/dave-foreign-data-introuction.html) > >> if anyone is interested). Now I'm being tasked to move it to Postgres > >> 9.2.x and I'd like to use FDW if possible. > >> > >> One of the problems I'm having is that in my application, the foreign > >> tables typically have hundreds of columns while typical queries only > >> access a dozen or so (the foreign server is a columnar SQL database). > >> Furthermore, there is no size optimization for NULL values passed back > >> from the foreign server, so if I return all of the columns from the table > >> --even as NULLs-- the returned data size will be several times the size > >> that it needs to be. My application cannot tolerate this level of > >> inefficiency, so I need to return minimal columns from the foreign table. > >> > >> The documentation doesn't say how to do this, but looking at the code I > >> think it is possible. In GetForeignPlan() you have to pass on the tlist > >> argument, which I presume means that the query plan will use the tlist > >> that I pass in, right? If so, then it should be possible for me to write a > >> function that takes tlist and baserel->reltargetlist and return a version > >> of tlist that knows which foreign-table columns are actually used, and > >> replaces the rest with a NULL constant. > >> > >> For example, suppose the original tlist is this: [VAR(attrno=1), > >> VAR(attrno=2), VAR(attrno=3)] and reltarget list says that I only need > >> args 1 and 3. Then the new tlist would look like this: [VAR(attrno=1), > >> CONST(val=NULL), VAR(attrno=2)] where the attrno of the last VAR has been > >> reduced by one because the 2 column is no longer there. > >> > >> I did something very much like this in my roll-your-own version of FDW so > >> I know basically how to do it, but I did it at the pre-planning stage and > >> I'm not sure how much is already packed into the other plan nodes at this > >> point. Maybe it's too late to change the target list? > >> > >> Can anyone give me some advice or warnings on this? I'd hate to go to the > >> trouble of implementing and testing it only to find that I'm making some > >> bogus assumptions. > >> > >> Thanks, > >> David Gudeman > >> > > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers -- David Fetter <da...@fetter.org> http://fetter.org/ Phone: +1 415 235 3778 AIM: dfetter666 Yahoo!: dfetter Skype: davidfetter XMPP: david.fet...@gmail.com iCal: webcal://www.tripit.com/feed/ical/people/david74/tripit.ics Remember to vote! Consider donating to Postgres: http://www.postgresql.org/about/donate -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
