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CREATE EXTENSION IF NOT EXISTS "pgcrypto";

-- Create a trigger function that takes no arguments.
-- Trigger functions automatically have OLD, NEW records
-- and TG_TABLE_NAME as well as others.
CREATE OR REPLACE FUNCTION unique_short_id()
RETURNS TRIGGER AS $$

 -- Declare the variables we'll be using.
DECLARE
  key TEXT;
  qry TEXT;
  found TEXT;
BEGIN

  -- generate the first part of a query as a string with safely
  -- escaped table name, using || to concat the parts
  qry := 'SELECT id FROM ' || quote_ident(TG_TABLE_NAME) || ' WHERE id=';

  -- This loop will probably only run once per call until we've generated
  -- millions of ids.
  LOOP

    -- Generate our string bytes and re-encode as a base64 string.
    key := encode(gen_random_bytes(6), 'base64');

    -- Base64 encoding contains 2 URL unsafe characters by default.
    -- The URL-safe version has these replacements.
    key := replace(key, '/', '_'); -- url safe replacement
    key := replace(key, '+', '-'); -- url safe replacement

    -- Concat the generated key (safely quoted) with the generated query
    -- and run it.
    -- SELECT id FROM "test" WHERE id='blahblah' INTO found
    -- Now "found" will be the duplicated id or NULL.
    EXECUTE qry || quote_literal(key) INTO found;

    -- Check to see if found is NULL.
    -- If we checked to see if found = NULL it would always be FALSE
    -- because (NULL = NULL) is always FALSE.
    IF found IS NULL THEN

      -- If we didn't find a collision then leave the LOOP.
      EXIT;
    END IF;

    -- We haven't EXITed yet, so return to the top of the LOOP
    -- and try again.
  END LOOP;

  -- NEW and OLD are available in TRIGGER PROCEDURES.
  -- NEW is the mutated row that will actually be INSERTed.
  -- We're replacing id, regardless of what it was before
  -- with our key variable.
  NEW.id = key;

  -- The RECORD returned here is what will actually be INSERTed,
  -- or what the next trigger will get if there is one.
  RETURN NEW;
END;
$$ language 'plpgsql';

CREATE TABLE test (id TEXT PRIMARY KEY, name TEXT);

-- We name the trigger "trigger_test_genid" so that we can remove
-- or replace it later.
-- If an INSERT contains multiple RECORDs, each one will call
-- unique_short_id individually.
CREATE TRIGGER trigger_test_genid BEFORE INSERT ON test FOR EACH ROW EXECUTE PROCEDURE unique_short_id();

INSERT INTO test (name) VALUES ('cheese'), ('ham'), ('turkey'), ('chicken');
SELECT * FROM test;

    id    |  name
----------+---------
 Ixw1yIj7 | cheese
 SXq0jZ-q | ham
 KKWXEtBu | turkey
 DRRXFs1U | chicken
(4 rows)