Re: minimizing the target list for foreign data wrappers - Mailing list pgsql-hackers

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.gudeman@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.gudeman@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
atthe time (some details here (http://unobtainabol.blogspot.com/2013/04/dave-foreign-data-introuction.html) if anyone
isinterested). 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
whiletypical queries only access a dozen or so (the foreign server is a columnar SQL database). Furthermore, there is
nosize 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
toleratethis 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()
youhave 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
returna 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
saysthat 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
itat the pre-planning stage and I'm not sure how much is already packed into the other plan nodes at this point. Maybe
it'stoo 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
onlyto find that I'm making some bogus assumptions.
 
> >>
> >> Thanks,
> >> David Gudeman
> >>
> 
> 
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