postgresql multiple statements in for loop

Question

I need to write a postgressql function to process some time-series data, which I would like to loop through each time step and decide whether the values in the few future time steps needs to be updated, and it will be the updated values that the decision is applied upon when the loop iterates to the following time steps. I hope I have explained it clearly.

Hence for example with the table:

CREATE TEMPORARY TABLE datatable (
                    unixdatetime integer,
                    value        integer,
                );

INSERT INTO datatable (unixdatetime, value) VALUES 
(1,56),
(2,23),
(3,7),
(4,68),
(5,31),
(6,42);

I have tried to apply the following for loop to it:

FOR r IN
     SELECT * FROM datatable
LOOP
   DROP TABLE IF EXISTS currentdata;

   CREATE TEMPORARY TABLE currentdata AS (
        SELECT dt.unixdatetime as currentTime,
               lead(dt.unixdatetime,1) OVER (ORDER BY dt.unixdatetime) AS lead1stTime,
               dt.value AS currentValue,
               lead(dt.value,1) OVER (ORDER BY dt.unixdatetime) AS lead1stVal
        FROM   datatable dt
   );

_counter = 0;

IF (SELECT currentValue%2 FROM currentdata) = 1
   THEN _counter = _counter + 1;
END IF;

IF (SELECT lead1stVal%2 FROM currentdata) = 1
   THEN _counter = _counter + 1;
END IF;

UPDATE datatable dt
SET    value = (CASE WHEN _counter = 2 AND cd.currentValue <> 888 THEN 999
                     ELSE cd.currentValue
               END)
FROM   currentdata cd
WHERE  dt.unixdatetime = cd.currentTime; 

UPDATE datatable dt
SET    value = (CASE WHEN _counter = 2 THEN 888
                     ELSE cd.lead1stVal
               END)
FROM   currentdata cd
WHERE  dt.unixdatetime = cd.lead1stTime;

END LOOP;

And my expected outcome would be:

| unixdatetime | Value |
|     1        |   56  |
|     2        |  999  |
|     3        |  888  |
|     4        |   68  |
|     5        |   31  |
|     6        |   42  |

As you all can see, I am still unable to switch into postgresql thinking mode, and still trying to use trick from languages such as python and C++ that doesn't work in postgressql. What I have noticed is that:

1. Each of these statements in the loop executes on the entire table before moving onto the next statement in the loop. instead on executing all statements sequentially before iterating onto the the row of the table.

2. Because of 1., the _counter does not provide the same effect like in the for loops of other languages. I do require the counter in the for loop as the actual task requires me to look into values more than 2 time steps ahead.

Any helpful suggestions of workarounds are appreciated.

Thanks in advance,

Jason

Answer 1

You can retrieve what you want without the need of a loop, using the following statement.

select unixdatetime, value,
       case  
          when is_odd and next_is_odd then 999 
          when is_odd and prev_is_odd then 888
          else value 
       end as new_value
from (
  select unixdatetime, value, 
         value % 2 <> 0 as is_odd,
         coalesce(lag(value) over (order by unixdatetime) % 2 <> 0, false) as prev_is_odd,
         coalesce(lead(value) over (order by unixdatetime) % 2 <> 0, false) as next_is_odd
  from datatable
) t
order by unixdatetime;

Now this can be used to directly update the whole table with a single statement:

update datatable
  set value = nv.new_value
from (
  select unixdatetime, value,
         case  
            when is_odd and next_is_odd then 999 
            when is_odd and prev_is_odd then 888
            else value 
         end as new_value
  from (
    select unixdatetime, value, 
           value % 2 <> 0 as is_odd,
           coalesce(lag(value) over (order by unixdatetime) % 2 <> 0, false) as prev_is_odd,
           coalesce(lead(value) over (order by unixdatetime) % 2 <> 0, false) as next_is_odd
    from datatable
  ) t
) nv
where nv.unixdatetime = datatable.unixdatetime
  and nv.new_value <> datatable.value; -- only update those that changed

SQLFiddle example: http://sqlfiddle.com/#!15/128d9/1