To ignore result in BEFORE TRIGGER of PostgreSQL?

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醉酒成梦
醉酒成梦 2021-01-15 06:07

This thread is a part challenge of this thread to which I am searching a better solution for one part by BEFORE TRIGGER. I just want to launch a trigger to convert to corre

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  • 2021-01-15 06:13

    First of all, you need to pass the row variable in a BEFORE trigger. Passing NULL cancels the operation for the row:

    CREATE OR REPLACE FUNCTION insbef_events_function() 
      RETURNS TRIGGER AS 
    $func$
    DECLARE
       m int[]; 
    BEGIN
       FOREACH m SLICE 1 IN ARRAY TG_ARGV[0]::int[]
       LOOP
          INSERT INTO events (measurement_id, event_index_start, event_index_end) 
          SELECT NEW.measurement_id, m[1], m[2];  -- Postgres array subscripts start with 1
       END LOOP;
    
          -- do something with _result ...
    
       RETURN NEW;  -- NULL would cancel operation in BEFORE trigger!
    END 
    $func$ LANGUAGE plpgsql;

    I demonstrated the use of RETRUN NULL in an AFTER trigger in my previous answer. You can't do the same for a BEFORE trigger. The documentation:

    Row-level triggers fired BEFORE can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the INSERT/UPDATE/DELETE does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value.

    There is more. Read the manual.

    But since you are passing two 1-dimensional arrays instead of one 2-dimensional array now, you need to adapt your trigger logic:

    CREATE OR REPLACE FUNCTION insbef_events_function() 
      RETURNS TRIGGER AS 
    $func$
    DECLARE
       a1 int[] := TG_ARGV[1]::int[];
       a2 int[] := TG_ARGV[2]::int[];
    BEGIN
       FOR i in array_lower(a1, 1) .. array_upper(a1, 1)
       LOOP
          INSERT INTO events (measurement_id, event_index_start, event_index_end) 
          SELECT NEW.measurement_id  -- or TG_ARGV[0]::int instead?
               , a1[i]
               , a2[i];
       END LOOP;
    
       RETURN NEW;  -- NULL would cancel operation in BEFORE trigger!
    END 
    $func$  LANGUAGE plpgsql;
    

    Now it's your responsibility that both arrays have the same number of elements.
    The function changing the trigger could look like this now:

    CREATE OR REPLACE FUNCTION f_create_my_trigger_events(_arg1 int, _arg2 text, _arg3 text)
      RETURNS void AS
    $func$
    BEGIN
       EXECUTE format(
        $$DROP TRIGGER IF EXISTS insbef_ids ON measurements; -- on measurements ..
          CREATE TRIGGER insbef_ids
          BEFORE INSERT ON measurements  -- .. according to previous posts!!
          FOR EACH ROW EXECUTE PROCEDURE insbef_events_function(%s, %L, %L)$$
        , _arg1
        , translate(_arg2, '[]', '{}')
        , translate(_arg3, '[]', '{}')
       );
    END
    $func$  LANGUAGE plpgsql;
    

    You need to understand basics of SQL, plpgsql and trigger functions before using this advanced automated design.

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