Re: [INTERFACES] remote database queries - Mailing list pgsql-hackers

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"Joe Conway" <joe.conway@mail.com> writes:
> Inspired by others who have recently gotten PostgreSQL functions to return
> sets, I set out to create my own. I have on more than one occasion wished I
> could run a query across databases or servers (similar to a dblink in Oracle
> or a linked db in MSSQL). Attached is a C file which is my attempt.

> select
>   dblink_tok(t1.f1,'~',0)::int as vl_id
>  ,dblink_tok(t1.f1,'~',1)::text as vl_guid
>  ,dblink_tok(t1.f1,'~',2)::text as vl_pri_email
>  ,dblink_tok(t1.f1,'~',3)::text as vl_acct_pass_phrase
>  ,dblink_tok(t1.f1,'~',4)::text as vl_email_relname
>  ,dblink_tok(t1.f1,'~',5)::text as vl_hwsn_relname
>  ,dblink_tok(t1.f1,'~',6)::timestamp as vl_mod_dt
>  ,dblink_tok(t1.f1,'~',7)::int as vl_status
> from
>  (select dblink('host=192.168.5.150 port=5432 dbname=vsreg_001 user=postgres
> password=postgres','select * from vs_lkup','~') as f1) as t1

> By doing "create view vs_lkup_rm as . . ." with the above query, from a
> database on another server, I can then write:
> "select * from vs_lkup" and get results just as if I were on 192.168.5.150
> (sort of -- see problem below).

> I have one question, and one problem regarding this.

> First the question: is there any way to get the dblink function to return
> setof composite -- i.e. return tuples instead of scalar values?

It could return a tuple, but there are notational problems that would
make it difficult to do anything useful with the tuple; in particular,
AFAICS you couldn't retrieve more than one column out of it, so there's
no point.

Until we fix that (maybe for 7.2, maybe not) your existing hack is
probably pretty reasonable.  You could save some cycles by avoiding
conversion to text, though --- instead return an opaque datum that is
pointer-to-tuple-slot and let the dblink_tok function extract fields
from the tuple.  Look at SQL function support and the FieldSelect
expression node type for inspiration.

> Now the problem: as I stated above, "select * from vs_lkup" returns results
> just as if I were on 192.168.5.150 -- but if I try "select * from vs_lkup
> WHERE vl_id = 1" or "select * from vs_lkup WHERE vl_pri_email in
> ('email1@foo.com')" I get the following error message: "ERROR:  Set-valued
> function called in context that cannot accept a set". Any ideas how to work
> around this?

I think this would work if the planner weren't so enthusiastic about
trying to collapse the sub-SELECT query together with the main query.
Unfortunately it doesn't check to see if any set-valued functions are
involved before it collapses 'em --- leaving you with a set-valued
function call in the WHERE clause.  Not sure if this is worth fixing,
considering that using set-valued functions in this way is just a
band-aid that doesn't have a long life expectancy.

If you just need a hack with a short life expectancy, here's a hack that
I recommend not reading right before dinner ... might make you lose your
appetite ;-).  Build the view as a dummy UNION:create view vs_lkup_rm as    select ... from (select dblink(...))
unionall    select null, null, ... where false;
 
Done this way, the UNION won't change the view's results --- but it will
prevent the (current version of the) planner from collapsing the view
together with the surrounding query.  For example:

regression=# create function dblink() returns setof int as
regression-# 'select f1 from int4_tbl' language 'sql';
CREATE
regression=# select dblink(); ?column?
-------------          0     123456    -123456 2147483647-2147483647
(5 rows)

regression=# create view vv1 as
regression-# select f1 from (select dblink() as f1) t1;
CREATE
regression=# select * from vv1 where f1 > 0;
ERROR:  Set-valued function called in context that cannot accept a set
regression=# create view vv2 as
regression-# select f1 from (select dblink() as f1) t1
regression-# union all
regression-# select null where false;
CREATE
regression=# select * from vv2;    f1
-------------          0     123456    -123456 2147483647-2147483647
(5 rows)

regression=# select * from vv2 where f1 > 0;    f1
------------    1234562147483647
(2 rows)

        regards, tom lane