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Migration to identity columns on PostgreSQL

As of version 4, the DBAL uses identity columns to implement the `autoincrement` behavior on PostgreSQL instead of `SERIAL*` column types.

If you have a database with `autoincrement` columns created using DBAL 3 or earlier, you will need to perform the following schema migration before being able to continue managing the schema with the DBAL:

  1. Identify all `autoincrement` columns and their tables.

  2. Create the upgrade_serial_to_identity()` function in the database as described in `PostgreSQL 10 identity columns explained:

    1CREATE OR REPLACE FUNCTION upgrade_serial_to_identity(tbl regclass, col name)
RETURNS void
LANGUAGE plpgsql
AS $$
DECLARE
  colnum smallint;
  seqid oid;
  count int;
BEGIN
  -- find column number
  SELECT attnum INTO colnum FROM pg_attribute WHERE attrelid = tbl AND attname = col;
  IF NOT FOUND THEN
    RAISE EXCEPTION 'column does not exist';
  END IF;

  -- find sequence
     SELECT INTO seqid objid
    FROM pg_depend
    WHERE (refclassid, refobjid, refobjsubid) = ('pg_class'::regclass, tbl, colnum)
      AND classid = 'pg_class'::regclass AND objsubid = 0
      AND deptype = 'a';

  GET DIAGNOSTICS count = ROW_COUNT;
  IF count < 1 THEN
    RAISE EXCEPTION 'no linked sequence found';
  ELSIF count > 1 THEN
    RAISE EXCEPTION 'more than one linked sequence found';
  END IF;

  -- drop the default
  EXECUTE 'ALTER TABLE ' || tbl || ' ALTER COLUMN ' || quote_ident(col) || ' DROP DEFAULT';

  -- change the dependency between column and sequence to internal
  UPDATE pg_depend
    SET deptype = 'i'
    WHERE (classid, objid, objsubid) = ('pg_class'::regclass, seqid, 0)
      AND deptype = 'a';

  -- mark the column as identity column
  UPDATE pg_attribute
    SET attidentity = 'd'
    WHERE attrelid = tbl
      AND attname = col;
END;
$$;
    

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  3. For each column and their table, run `upgrade_serial_to_identity(<table>, <column>)`.

Without this migration, next time when DBAL 4 is used to manage the schema, it will perform a similar migration but instead of reusing the existing sequence, it will drop it and create a new one. As a result, all new sequence numbers will be generated from `1`, which is most likely undesired.