create or replace function pgq.batch_event_tables(x_batch_id bigint)
returns setof text as $$
-- ----------------------------------------------------------------------
-- Function: pgq.batch_event_tables(1)
--
--     Returns set of table names where this batch events may reside.
--
-- Parameters:
--     x_batch_id    - ID of a active batch.
-- ----------------------------------------------------------------------
declare
    nr                    integer;
    tbl                   text;
    use_prev              integer;
    use_next              integer;
    batch                 record;
begin
    select
           txid_snapshot_xmin(last.tick_snapshot) as tx_min, -- absolute minimum
           txid_snapshot_xmax(cur.tick_snapshot) as tx_max, -- absolute maximum
           q.queue_data_pfx, q.queue_ntables,
           q.queue_cur_table, q.queue_switch_step1, q.queue_switch_step2
        into batch
        from pgq.tick last, pgq.tick cur, pgq.subscription s, pgq.queue q
        where cur.tick_id = s.sub_next_tick
          and cur.tick_queue = s.sub_queue
          and last.tick_id = s.sub_last_tick
          and last.tick_queue = s.sub_queue
          and s.sub_batch = x_batch_id
          and q.queue_id = s.sub_queue;
    if not found then
        raise exception 'Cannot find data for batch %', x_batch_id;
    end if;

    -- if its definitely not in one or other, look into both
    if batch.tx_max < batch.queue_switch_step1 then
        use_prev := 1;
        use_next := 0;
    elsif batch.queue_switch_step2 is not null
      and (batch.tx_min > batch.queue_switch_step2)
    then
        use_prev := 0;
        use_next := 1;
    else
        use_prev := 1;
        use_next := 1;
    end if;

    if use_prev then
        nr := batch.queue_cur_table - 1;
        if nr < 0 then
            nr := batch.queue_ntables - 1;
        end if;
        tbl := batch.queue_data_pfx || '_' || nr::text;
        return next tbl;
    end if;

    if use_next then
        tbl := batch.queue_data_pfx || '_' || batch.queue_cur_table::text;
        return next tbl;
    end if;

    return;
end;
$$ language plpgsql; -- no perms needed