Thread: Returning a reference to a cursor from a function

On Thursday 12 Sep 2002 7:12 pm, david williams wrote:
> To anyone who can help me,
>
> I am new at Postgresql and am having some problems.

> I went a stage further attempt to put this query into a function as such
> CREATE FUNCTION getallusers() RETURN integer AS'
> DECLARE
>     Liahona CURSOR FOR Select * from users;
> BEGIN
>
> FETCH FORWARD ALL IN Liahona;
>
> CLOSE Liahona;
> END;
> 'language 'plpgsql';

Number of issues here - it's RETURNS on the first line, you haven't OPENed the
cursor and I don't think you can use that form of FETCH in the function. Look
at the Programmers Guide, ch 23.7 "Cursors" for details on how to do it.

To get you going, here's one that returns a count of how many records were
fetched from an indicated table.

-- BEGIN function --
DROP FUNCTION foo_count(text);
CREATE FUNCTION foo_count(text) RETURNS integer AS '
DECLARE   my_name ALIAS FOR $1;
   csr1 refcursor;   dummy RECORD;   n int4;
BEGIN   n:=0;   RAISE NOTICE ''counting table: %'',my_name;   OPEN csr1 FOR EXECUTE ''SELECT * FROM '' || my_name;
FETCHcsr1 INTO dummy;   WHILE (FOUND) LOOP       n:=n+1;       FETCH csr1 INTO dummy;   END LOOP;   CLOSE csr1; 
   RETURN n;
END;
'language 'plpgsql';
-- END function --

Put this in a text-file and use \i filename from psql to import the definition
- makes it easier to debug.

On Tuesday 17 Sep 2002 7:12 pm, you wrote:
> Richard,
>
> Thanks for the information. I've made some modifications to your code here
> so that it does a RAISE NOTICE in each loop returning simply the value of n
> and then when the loop is finished it again returns n.
>
> This works fine at the psql level but after it passes through ODBC to the
> ASP layer all I get is the final RETURN value.

Yep - the NOTICE is really a type of error message (you can use RAISE to
generate errors too) and isn't part of your data-stream.

> I have tried using the RETURN function in the loop but it terminates the
> loop.

Indeed it does.

> I really need to return each record up to the ASP layer.

The solution to this sort of thing in version 7.3 is something called table
functions, but I think they're limited to C at the moment, not plpgsql.

With 7.2 you need to return the cursor from the function and then FETCH from
it. An example was missed out from the 7.2.1 docs but you can see one in the
developer's docs (bottom of page):
http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html

To hack our example a bit more the below takes a cursor-name and table name
and defines a cursor for you.
Note that when using it, you need to be within BEGIN...COMMIT (an explicit
transaction) since the cursor returned from the function only lasts until the
end of a transaction.

HTH

- Richard Huxton

DROP FUNCTION foo_count(refcursor, text);
CREATE FUNCTION foo_count(refcursor, text) RETURNS refcursor AS '
DECLARE   curs ALIAS FOR $1;   tbl_name ALIAS FOR $2;
BEGIN   RAISE NOTICE ''cursor on table: %'',tbl_name;   OPEN curs FOR EXECUTE ''SELECT * FROM '' || tbl_name;   RETURN
curs;
END;
'language 'plpgsql';

richardh=> BEGIN;
BEGIN
richardh=> SELECT foo_count('fake_cursor','companies');
NOTICE:  cursor on table: companies foo_count
-------------fake_cursor
(1 row)

richardh=> FETCH 3 FROM fake_cursor;co_id |         co_name          | co_postcode |          co_lastchg
-------+--------------------------+-------------+-------------------------------   56 | Acme Associates GmBH     |
unknown    | 2002-06-12  
14:04:43.123408+01   57 | Imperial Investments Inc | unknown     | 2002-06-12
14:04:43.123408+01   58 | Universal Associates USA | unknown     | 2002-06-12
14:04:43.123408+01
(3 rows)

richardh=> COMMIT;

----- Original Message -----

From: Richard Huxton

Sent: Wednesday, September 18, 2002 6:32 AM

To: david williams

Cc: pgsql-sql@postgresql.org

Subject: Re: [SQL] Returning a reference to a cursor from a function

 

On Tuesday 17 Sep 2002 7:12 pm, you wrote:
> Richard,
>
> Thanks for the information. I've made some modifications to your code here
> so that it does a RAISE NOTICE in each loop returning simply the value of n
> and then when the loop is finished it again returns n.
>
> This works fine at the psql level but after it passes through ODBC to the
> ASP layer all I get is the final RETURN value.

Yep - the NOTICE is really a type of error message (you can use RAISE to
generate errors too) and isn't part of your data-stream.

> I have tried using the RETURN function in the loop but it terminates the
> loop.

Indeed it does.

> I really need to return each record up to the ASP layer.

The solution to this sort of thing in version 7.3 is something called table
functions, but I think they're limited to C at the moment, not plpgsql.

With 7.2 you need to return the cursor from the function and then FETCH from
it. An example was missed out from the 7.2.1 docs but you can see one in the
developer's docs (bottom of page):
http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html

To hack our example a bit more the below takes a cursor-name and table name
and defines a cursor for you.
Note that when using it, you need to be within BEGIN...COMMIT (an explicit
transaction) since the cursor returned from the function only lasts until the
end of a transaction.

HTH

- Richard Huxton

DROP FUNCTION foo_count(refcursor, text);
CREATE FUNCTION foo_count(refcursor, text) RETURNS refcursor AS '
DECLARE
    curs ALIAS FOR $1;
    tbl_name ALIAS FOR $2;
BEGIN
    RAISE NOTICE ''cursor on table: %'',tbl_name;
    OPEN curs FOR EXECUTE ''SELECT * FROM '' || tbl_name;
    RETURN curs;
END;
'language 'plpgsql';

richardh=> BEGIN;
BEGIN
richardh=> SELECT foo_count('fake_cursor','companies');
NOTICE:  cursor on table: companies
  foo_count
-------------
fake_cursor
(1 row)

richardh=> FETCH 3 FROM fake_cursor;
co_id |         co_name          | co_postcode |          co_lastchg
-------+--------------------------+-------------+-------------------------------
    56 | Acme Associates GmBH     | unknown     | 2002-06-12
14:04:43.123408+01
    57 | Imperial Investments Inc | unknown     | 2002-06-12
14:04:43.123408+01
    58 | Universal Associates USA | unknown     | 2002-06-12
14:04:43.123408+01
(3 rows)

richardh=> COMMIT;

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