Обсуждение: cursors and function question

Hi

Is there any elegant way not a two steps way I can output the cursor value at each step?


testtbl table has this content

    col1    |    col2    | col3
------------+------------+------
 E1         | CAT1       |    0
 E1         | CAT2       |    0
 E1         | CAT3       |    0
 E4         | CAT1       |    0
 E5         | CAT1       |    0
 E6         | CAT1       |    0
 E7         | CAT1       |    0


This works
BEGIN WORK;
DECLARE fooc  CURSOR FOR SELECT * FROM testtbl;
FETCH ALL FROM fooc;
CLOSE fooc;
COMMIT WORK;

    col1    |    col2    | col3
------------+------------+------
 E1         | CAT1       |    0
 E1         | CAT2       |    0
 E1         | CAT3       |    0
 E4         | CAT1       |    0
 E5         | CAT1       |    0
 E6         | CAT1       |    0
 E7         | CAT1       |    0


But
CREATE OR REPLACE FUNCTION foofunc()
   RETURNS text AS $$
DECLARE
 var2   RECORD;
 cur CURSOR FOR SELECT * from testtbl;
BEGIN
   OPEN cur;
    LOOP
      FETCH cur INTO var2;
      return var2;
   END LOOP;
   CLOSE cur;
END; $$
LANGUAGE plpgsql;


select foofunc();
            foofunc
-------------------------------
 ("E1        ","CAT1      ",0)

But I am looking to get

            foofunc
-------------------------------
 ("E1        ","CAT1      ",0)
 ("E1        ","CATs      ",0)
etc



Many thanks
— Armand

CREATE OR REPLACE FUNCTION foofunc()
   RETURNS text AS $$

select foofunc();
            foofunc
-------------------------------
 ("E1        ","CAT1      ",0)

But I am looking to get

            foofunc
-------------------------------
 ("E1        ","CAT1      ",0)
 ("E1        ","CATs      ",0)

On 02/13/2018 10:22 AM, armand pirvu wrote:
> Hi
> 
> Is there any elegant way not a two steps way I can output the cursor value at each step?
> 
> 
> testtbl table has this content
> 
>      col1    |    col2    | col3
> ------------+------------+------
>   E1         | CAT1       |    0
>   E1         | CAT2       |    0
>   E1         | CAT3       |    0
>   E4         | CAT1       |    0
>   E5         | CAT1       |    0
>   E6         | CAT1       |    0
>   E7         | CAT1       |    0
> 
> 
> This works
> BEGIN WORK;
> DECLARE fooc  CURSOR FOR SELECT * FROM testtbl;
> FETCH ALL FROM fooc;
> CLOSE fooc;
> COMMIT WORK;
> 
>      col1    |    col2    | col3
> ------------+------------+------
>   E1         | CAT1       |    0
>   E1         | CAT2       |    0
>   E1         | CAT3       |    0
>   E4         | CAT1       |    0
>   E5         | CAT1       |    0
>   E6         | CAT1       |    0
>   E7         | CAT1       |    0
> 
> 
> But
> CREATE OR REPLACE FUNCTION foofunc()
>     RETURNS text AS $$
> DECLARE
>   var2   RECORD;
>   cur CURSOR FOR SELECT * from testtbl;
> BEGIN
>     OPEN cur;
>      LOOP
>        FETCH cur INTO var2;
>        return var2;
>     END LOOP;
>     CLOSE cur;
> END; $$
> LANGUAGE plpgsql;


CREATE OR REPLACE FUNCTION public.foofunc()
  RETURNS SETOF testtbl
  LANGUAGE sql
AS $function$
     SELECT * FROM testtbl;
$function$


test=> select * from foofunc();
  col1 | col2 | col3
------+------+------
  E1   | CAT1 |    0
  E1   | CAT2 |    0
  E1   | CAT3 |    0
  E4   | CAT1 |    0
  E5   | CAT1 |    0
  E6   | CAT1 |    0
  E7   | CAT1 |    0
(7 rows)


> 
> 
> select foofunc();
>              foofunc
> -------------------------------
>   ("E1        ","CAT1      ",0)
> 
> But I am looking to get
> 
>              foofunc
> -------------------------------
>   ("E1        ","CAT1      ",0)
>   ("E1        ","CATs      ",0)
> etc
> 
> 
> 
> Many thanks
> — Armand
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 02/13/2018 11:17 AM, armand pirvu wrote:
> 
>> On Feb 13, 2018, at 12:54 PM, Adrian Klaver <adrian.klaver@aklaver.com 
>> <mailto:adrian.klaver@aklaver.com>> wrote:
>>
>> On 02/13/2018 10:22 AM, armand pirvu wrote:
>>> Hi
>>> Is there any elegant way not a two steps way I can output the cursor 
>>> value at each step?
>>> testtbl table has this content
>>>     col1    |    col2    | col3
>>> ------------+------------+------
>>>  E1         | CAT1       |    0
>>>  E1         | CAT2       |    0
>>>  E1         | CAT3       |    0
>>>  E4         | CAT1       |    0
>>>  E5         | CAT1       |    0
>>>  E6         | CAT1       |    0
>>>  E7         | CAT1       |    0
>>> This works
>>> BEGIN WORK;
>>> DECLARE fooc  CURSOR FOR SELECT * FROM testtbl;
>>> FETCH ALL FROM fooc;
>>> CLOSE fooc;
>>> COMMIT WORK;
>>>     col1    |    col2    | col3
>>> ------------+------------+------
>>>  E1         | CAT1       |    0
>>>  E1         | CAT2       |    0
>>>  E1         | CAT3       |    0
>>>  E4         | CAT1       |    0
>>>  E5         | CAT1       |    0
>>>  E6         | CAT1       |    0
>>>  E7         | CAT1       |    0
>>> But
>>> CREATE OR REPLACE FUNCTION foofunc()
>>>    RETURNS text AS $$
>>> DECLARE
>>>  var2   RECORD;
>>>  cur CURSOR FOR SELECT * from testtbl;
>>> BEGIN
>>>    OPEN cur;
>>>     LOOP
>>>       FETCH cur INTO var2;
>>>       return var2;
>>>    END LOOP;
>>>    CLOSE cur;
>>> END; $$
>>> LANGUAGE plpgsql;
>>
>>
>> CREATE OR REPLACE FUNCTION public.foofunc()
>> RETURNS SETOF testtbl
>> LANGUAGE sql
>> AS $function$
>>    SELECT * FROM testtbl;
>> $function$
>>
>>
>> test=> select * from foofunc();
>> col1 | col2 | col3
>> ------+------+------
>> E1   | CAT1 |    0
>> E1   | CAT2 |    0
>> E1   | CAT3 |    0
>> E4   | CAT1 |    0
>> E5   | CAT1 |    0
>> E6   | CAT1 |    0
>> E7   | CAT1 |    0
>> (7 rows)
>>
>>
>>> select foofunc();
>>>             foofunc
>>> -------------------------------
>>>  ("E1        ","CAT1      ",0)
>>> But I am looking to get
>>>             foofunc
>>> -------------------------------
>>>  ("E1        ","CAT1      ",0)
>>>  ("E1        ","CATs      ",0)
>>> etc
>>> Many thanks
>>> — Armand
>>
>>
>> --
>> Adrian Klaver
>> adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
> 
> 
> 
> Thanks Adrian
> 
> That one I figured it out as well. The idea is that said table has some 
> records which I need to loop and do some processing using cursors 
> similar with
> 
> DECLARE
> cur CURSOR FOR SELECT *
>      FROM testtbl FOR UPDATE;
> BEGIN
>    FOR row IN cur LOOP
>      UPDATE testtbl
>      SET col3=1
>      WHERE CURRENT OF cur;
>    END LOOP;
>    return cur;
> END
> 
> For a row update the goal is to return the cursor value  be it 
> before/after the update, hence my question and test

Not following, are you looking to do this in an UPDATE trigger or 
somewhere else?

Another way to ask is why do you want to use a cursor?

> 
> I found some code which seems to do what I need but it involves two 
> functions
> CREATE or replace FUNCTION reffunc(refcursor) RETURNS refcursor AS $$
> BEGIN
>      OPEN $1 FOR SELECT col FROM test;
>      RETURN $1;
> END;
> $$ LANGUAGE plpgsql;
> 
> BEGIN;
> SELECT reffunc('funccursor');
> FETCH ALL IN funccursor;
> COMMIT;
> 
> 
> And this is what beats  me , aka can I put all in one / how ?
> 
> 
> 
> 
> Thanks
> Armand
> 
> 
> 
> 
> 
> 
> 
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 02/13/2018 01:25 PM, armand pirvu wrote:
> 
>> On Feb 13, 2018, at 1:22 PM, Adrian Klaver <adrian.klaver@aklaver.com 
>> <mailto:adrian.klaver@aklaver.com>> wrote:
>>

> 
> Not a trigger , but the idea is we will do some batch processing from 
> said table let’s name it testtbl
> 
> 1 - we get the records using  select for update with a limit 100 for example
> 2 - update each record using using cursor
> 3 - print the cursor content so that way I have an idea what was updated
> 
> I was thinking that if I can put a unique constraint on the table, I can 
> generate a global table in the function , update main table from global 
> table and return select from global table

Not entirely sure I know what you are trying to accomplish, still:

1) Not sure you need to use cursor, see here for less complicated way:

https://www.postgresql.org/docs/10/static/plpgsql-control-structures.html#PLPGSQL-RECORDS-ITERATING

and

https://www.postgresql.org/docs/10/static/plpgsql-control-structures.html#PLPGSQL-STATEMENTS-RETURNING

Using RETURN NEXT.

Keeping mind:

https://www.postgresql.org/docs/10/static/plpgsql-cursors.html
"Rather than executing a whole query at once, it is possible to set up a 
cursor that encapsulates the query, and then read the query result a few 
rows at a time. One reason for doing this is to avoid memory overrun 
when the result contains a large number of rows. (However, PL/pgSQL 
users do not normally need to worry about that, since FOR loops 
automatically use a cursor internally to avoid memory problems.) A more 
interesting usage is to return a reference to a cursor that a function 
has created, allowing the caller to read the rows. This provides an 
efficient way to return large row sets from functions."

So if you are keeping the rows to 100 a FOR loop would seem to suffice.

2) By global table do you mean a temporary table? If so not sure that is 
going to work as I am pretty sure it will disappear after the function 
is run. I could see having a permanent table that you INSERT the updated 
rows into with a timestamp. Then you could update the main table from 
that and prune old records using the timestamps.


> 
> I can see the developer desire to use cursors to minimize some effort on 
> his side
> 
> Thanks
> 
> Armand
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com