gistfile1.txt

-- Schema version ---------------------------------------------------------

create table schema_version (
  version integer not null
);
create unique index schema_version_one_row on schema_version((1));
create function schema_version_no_delete () returns trigger as $f$
begin
  raise exception 'Cannot delete schema version';
end;
$f$ language plpgsql;
create trigger schema_version_no_delete before delete on schema_version for each row execute procedure schema_version_no_delete();

create function assert_schema_version(expected integer) returns boolean as $f$
declare
  ver schema_version%ROWTYPE;
begin
  begin
    select * into strict ver from schema_version;
    exception
      when NO_DATA_FOUND then
        raise exception 'Version not found in schema_version';
      when TOO_MANY_ROWS then
        raise exception 'schema_version malformed';
  end;
  if ver.version <> $1 then
    raise exception 'This patch can only be applied to schema version % not %', $1, ver.version;
  end if;
  return true;
end;
$f$ language plpgsql;

insert into schema_version (version) values (1);