create or replace function pgq.maint_rotate_tables_step1(i_queue_name text)
returns integer as $$
-- ----------------------------------------------------------------------
-- Function: pgq.maint_rotate_tables_step1(1)
--
--      Rotate tables for one queue.
--
-- Parameters:
--      i_queue_name        - Name of the queue
--
-- Returns:
--      0
-- ----------------------------------------------------------------------
declare
    badcnt          integer;
    cf              record;
    nr              integer;
    tbl             text;
    lowest_tick_id  int8;
    lowest_xmin     int8;
begin
    -- check if needed and load record
    select * from pgq.queue into cf
        where queue_name = i_queue_name
          and queue_rotation_period is not null
          and queue_switch_step2 is not null
          and queue_switch_time + queue_rotation_period < current_timestamp
        for update;
    if not found then
        return 0;
    end if;

    -- if DB is in invalid state, stop
    if txid_current() < cf.queue_switch_step1 then
        raise exception 'queue % maint failure: step1=%, current=%',
                i_queue_name, cf.queue_switch_step1, txid_current();
    end if;

    -- find lowest tick for that queue
    select min(sub_last_tick) into lowest_tick_id
      from pgq.subscription
     where sub_queue = cf.queue_id;

    -- if some consumer exists
    if lowest_tick_id is not null then
        -- is the slowest one still on previous table?
        select txid_snapshot_xmin(tick_snapshot) into lowest_xmin
          from pgq.tick
         where tick_queue = cf.queue_id
           and tick_id = lowest_tick_id;
        if not found then
            raise exception 'queue % maint failure: tick % not found', i_queue_name, lowest_tick_id;
        end if;
        if lowest_xmin <= cf.queue_switch_step2 then
            return 0; -- skip rotation then
        end if;
    end if;

    -- nobody on previous table, we can rotate
    
    -- calc next table number and name
    nr := cf.queue_cur_table + 1;
    if nr = cf.queue_ntables then
        nr := 0;
    end if;
    tbl := cf.queue_data_pfx || '_' || nr::text;

    -- there may be long lock on the table from pg_dump,
    -- detect it and skip rotate then
    begin
        execute 'lock table ' || pgq.quote_fqname(tbl) || ' nowait';
        execute 'truncate ' || pgq.quote_fqname(tbl);
    exception
        when lock_not_available then
            -- cannot truncate, skipping rotate
            return 0;
    end;

    -- remember the moment
    update pgq.queue
        set queue_cur_table = nr,
            queue_switch_time = current_timestamp,
            queue_switch_step1 = txid_current(),
            queue_switch_step2 = NULL
        where queue_id = cf.queue_id;

    -- Clean ticks by using step2 txid from previous rotation.
    -- That should keep all ticks for all batches that are completely
    -- in old table.  This keeps them for longer than needed, but:
    -- 1. we want the pgq.tick table to be big, to avoid Postgres
    --    accitentally switching to seqscans on that.
    -- 2. that way we guarantee to consumers that they an be moved
    --    back on the queue at least for one rotation_period.
    --    (may help in disaster recovery)
    delete from pgq.tick
        where tick_queue = cf.queue_id
          and txid_snapshot_xmin(tick_snapshot) < cf.queue_switch_step2;

    return 0;
end;
$$ language plpgsql; -- need admin access


create or replace function pgq.maint_rotate_tables_step2()
returns integer as $$
-- ----------------------------------------------------------------------
-- Function: pgq.maint_rotate_tables_step2(0)
--
--      Stores the txid when the rotation was visible.  It should be
--      called in separate transaction than pgq.maint_rotate_tables_step1()
-- ----------------------------------------------------------------------
begin
    update pgq.queue
       set queue_switch_step2 = txid_current()
     where queue_switch_step2 is null;
    return 0;
end;
$$ language plpgsql; -- need admin access