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unique_short_id.sql
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unique_short_id.sql
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-- This script creates a trigger that generates a 6-byte hex string.
-- https://blog.andyet.com/2016/02/23/generating-shortids-in-postgres/
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 $$
-- Declare the variables we'll be using.
DECLARE
key TEXT;
qry TEXT;
found TEXT;
BEGIN
-- generate the first part of a query as a string with safely
-- escaped table name, using || to concat the parts
qry := 'SELECT id FROM ' || quote_ident(TG_TABLE_NAME) || ' WHERE id=';
-- This loop will probably only run once per call until we've generated
-- millions of ids.
LOOP
-- Generate our string bytes and re-encode as a base64 string.
key := encode(gen_random_bytes(6), 'hex');
-- Concat the generated key (safely quoted) with the generated query
-- and run it.
-- SELECT id FROM "test" WHERE id='blahblah' INTO found
-- Now "found" will be the duplicated id or NULL.
EXECUTE qry || quote_literal(key) INTO found;
-- Check to see if found is NULL.
-- If we checked to see if found = NULL it would always be FALSE
-- because (NULL = NULL) is always FALSE.
IF found IS NULL THEN
-- If we didn't find a collision then leave the LOOP.
EXIT;
END IF;
-- We haven't EXITed yet, so return to the top of the LOOP
-- and try again.
END LOOP;
-- NEW and OLD are available in TRIGGER PROCEDURES.
-- NEW is the mutated row that will actually be INSERTed.
-- We're replacing id, regardless of what it was before
-- with our key variable.
NEW.id = key;
-- The RECORD returned here is what will actually be INSERTed,
-- or what the next trigger will get if there is one.
RETURN NEW;
END;
$$ language 'plpgsql';