PostgreSQL-simple `execute` on a function fails with "execute resulted in Col 1-column result"

Question

I want to execute a Postgres function from Haskell which updates 3 rows but is declared with RETURNS VOID. I run the function as follows:

catch (do execute conn "select record(?,?)" [id1, id2])
      (\(e :: SomeException) -> do putStrLn ("Exception:" ++ (show e)); return False)

but this results in:

QueryError {qeMessage = "execute resulted in Col 1-column result", qeQuery = "select record(?,?)"}

The query doesn't return results:

ebdb=> select record('','');
 record
--------------------

(1 row)

How can I execute this Postgresql function from Haskell?

Answer 1

This tricky query returns no rows, still the function is executed:

select 1 where record('', '') isnull;

Answer 2

I would try using query instead of execute:

query conn "select 1 from record(?,?)" [id1, id2]

execute is for statements like INSERT, UPDATE, etc. Even though your statement does not return any rows, it is still a SELECT so I think you need to use query to run it.

Answer 3

I got this error and found this question but wasn't able to switch to query because I'm using a library that uses execute for database migrations. (I needed an empty/no-op migration.)

Here's what I ended up using which worked:

UPDATE arbitrary_but_real_table_name
  SET arbitrary_text_column = 'this will not happen' 
  WHERE 'this is not null' IS NULL;

Hope this helps someone!