update multidimensional arrays in postgresql - arrays

Question

id Weather
1 {{KS,'S'},{MO,'S'},{CA,'S'}}
I am trying to update 'S' to 'W' for all KS,MO,CA.
I am executing below query and it is throwing me an error
UPDATE table
SET Weather[][2] ='W' where id=1;
expected output
id Weather
1 {{KS,'W'},{MO,'W'},{CA,'W'}}

Answer 1

I think is possible using ordinary UPDATE.
Let's try to select items what we need to replace:
-- first, second and third item from array and convert to string
SELECT array_to_string(weather[1:1], ',') AS KS,
array_to_string(weather[2:2], ',') AS MO,
array_to_string(weather[3:3], ',') AS CA
FROM test_tbl;
Result: "KS,'S'","MO,'S'","CA,'S'"
Now we can select records records for update using just comparing string with array item(as string):
SELECT * FROM test_tbl
-- KS,'S'
WHERE array_to_string(weather[1:1], ',') = concat('KS,', quote_literal('S'))
-- MO,'S'
-- array_to_string(weather[2:2], ',') = concat('MO,', quote_literal('S'))
-- CA,'S'
-- array_to_string(weather[3:3], ',') = concat('CA,', quote_literal('S'))
Ok. Now we need just to merge array by parts with a new item.
UPDATE test_tbl
-- generate first item + other items
SET weather = string_to_array(concat('KS,', quote_literal('W')), ',')::varchar[] || weather[2:]
WHERE array_to_string(weather[1:1], ',') = concat('KS,', quote_literal('S'));
UPDATE test_tbl
-- first item + generate second + other items
SET weather = weather[1:1] || string_to_array(concat('MO,', quote_literal('W')), ',')::varchar[] || weather[3:]
WHERE array_to_string(weather[2:2], ',') = concat('MO,', quote_literal('S'));
UPDATE test_tbl
-- first + second items, generate third + other items
SET weather = weather[0:2] || string_to_array(concat('CA,', quote_literal('W')), ',')::varchar[] || weather[4:]
WHERE array_to_string(weather[3:3], ',') = concat('CA,', quote_literal('S'));
Result: {{KS,'W'},{MO,'W'},{CA,'W'}}. Hope this helps

Answer 2

I believe that normalizing your data would be the best solution, but if it is not an option for you, try this function:
CREATE OR REPLACE FUNCTION change_array(p TEXT[][]) RETURNS TEXT[][] AS $$
DECLARE row record; res TEXT[][];
DECLARE i INT :=0;
BEGIN
LOOP
EXIT WHEN i = array_length(p,1);
i:=i+1;
IF p[i:i][1:2] <# ARRAY[['KS','S']] THEN
res := res || ARRAY[['KS','W']];
ELSEIF p[i:i][1:2] <# ARRAY[['MO','S']] THEN
res := res || ARRAY[['MO','W']];
ELSEIF p[i:i][1:2] <# ARRAY[['CA','S']] THEN
res := res || ARRAY[['CA','W']];
ELSE res := res || p[i:i];
END IF;
END LOOP;
RETURN res;
END;
$$ LANGUAGE plpgsql ;
Testing with your example..
SELECT change_array(ARRAY[['KS','S'],['MO','S'],['CA','S'],['XX','S']]);
change_array
-------------------------------
{{KS,W},{MO,W},{CA,W},{XX,S}}
(1 Zeile)