Pager usage is off.
Timing is on.
SET
Time: 78.776 ms
SET
Time: 81.274 ms
DO $$
DECLARE
  tbl text        := quote_ident('entity_associations');
  target_col text := quote_ident('target_entity_id');
  new_col text    := quote_ident('tmp_promote_target_entity_id');

BEGIN
  -- If the target column does not exist, fail.

  IF NOT EXISTS (
    SELECT 1 FROM information_schema.columns
      WHERE table_name = tbl
        AND column_name = target_col
    ) THEN
    RAISE EXCEPTION 'target column % does not exist', target_col;
  END IF;

  -- If the target column exists, but is not of type integer, fail.

  IF NOT EXISTS (
    SELECT 1 FROM information_schema.columns
      WHERE table_name = tbl
        AND column_name = target_col
        AND data_type = 'integer'
    ) THEN
    RAISE EXCEPTION 'target column % must have type integer', target_col;
  END IF;

  -- If the new column already exists, fail.

  -- NB: It is intentional that a user must deliberately delete a leftover new
  -- column from a previous, failed execution of this script. Automatic cleanup
  -- could result in catastrophe if the arguments to target_col and new_col were
  -- accidentally swapped!

  IF EXISTS (
    SELECT 1 FROM information_schema.columns
      WHERE table_name = tbl
        AND column_name = new_col
    ) THEN
    RAISE EXCEPTION
      'Temp column % already exists! To continue, confirm that this is in fact a temp column, and remove it.',
      new_col;
  END IF;
END
$$;
DO
Time: 94.230 ms
SELECT run_command_on_workers(
  $cmd$

  CREATE OR REPLACE FUNCTION copy_target_entity_id_to_tmp_promote_target_entity_id_on_entity_associations()
    RETURNS TRIGGER AS
  $copy_target_entity_id_to_tmp_promote_target_entity_id_on_entity_associations$
  BEGIN
    NEW.tmp_promote_target_entity_id := NEW.target_entity_id;
    RETURN NEW;
  END;
  $copy_target_entity_id_to_tmp_promote_target_entity_id_on_entity_associations$ LANGUAGE plpgsql;
  $cmd$
);
                        run_command_on_workers
-----------------------------------------------------------------------
 (ec2-18-210-81-75.compute-1.amazonaws.com,5432,t,"CREATE FUNCTION")
 (ec2-18-232-211-178.compute-1.amazonaws.com,5432,t,"CREATE FUNCTION")
(2 rows)

Time: 84.638 ms
ALTER TABLE entity_associations ADD COLUMN tmp_promote_target_entity_id bigint;
ALTER TABLE
Time: 90.755 ms
SELECT run_command_on_shards(
  'entity_associations',
  $cmd$
  DROP TRIGGER IF EXISTS backfill_target_entity_id_to_tmp_promote_target_entity_id_on_entity_associations ON %s
  $cmd$
);
   run_command_on_shards
---------------------------
 (102224,t,"DROP TRIGGER")
 (102225,t,"DROP TRIGGER")
 (102226,t,"DROP TRIGGER")
 (102227,t,"DROP TRIGGER")
 (102228,t,"DROP TRIGGER")
 (102229,t,"DROP TRIGGER")
 (102230,t,"DROP TRIGGER")
 (102231,t,"DROP TRIGGER")
(8 rows)

Time: 91.372 ms
SELECT run_command_on_shards('entity_associations', $cmd$ CREATE TRIGGER backfill_target_entity_id_to_tmp_promote_target_entity_id_on_entity_associations
  BEFORE INSERT OR UPDATE ON %s
  FOR EACH ROW
  EXECUTE PROCEDURE copy_target_entity_id_to_tmp_promote_target_entity_id_on_entity_associations (target_entity_id, tmp_promote_target_entity_id); $cmd$
);
    run_command_on_shards
-----------------------------
 (102224,t,"CREATE TRIGGER")
 (102225,t,"CREATE TRIGGER")
 (102226,t,"CREATE TRIGGER")
 (102227,t,"CREATE TRIGGER")
 (102228,t,"CREATE TRIGGER")
 (102229,t,"CREATE TRIGGER")
 (102230,t,"CREATE TRIGGER")
 (102231,t,"CREATE TRIGGER")
(8 rows)

Time: 86.881 ms
Done!