Question

I am building a FIFO queue using postgres and the answer based on apinstein answer to Job queue as SQL table with multiple consumers (PostgreSQL)

The issue is, if I use a transaction so the statement looks like:

    begin;
lock table queued_message in exclusive mode;
update 
    queued_message
set 
    status='IN_PROGRESS'
where
    id in (
        select
            id
        from
            queued_message
        where
            status='SUBMITTED' and queue='BACKFILL_REQUEST'
        order by 
            id asc
        limit 1
    )
returning *;
commit;

Then my returning values get discarded. If I run the same statement without the begin/commit the recordset returns fine.

Obviously I would prefer the transaction; the statement may not even be safe without it. So how do I return my committed recordeset?

EDIT
I’m marking the answer since it got me on the right track, but here is the function I ended up with:

CREATE TYPE returned_message as (id bigint, body json, status character varying(50) , queue character varying(150), last_modified timestamp without time zone)

CREATE OR REPLACE FUNCTION get_next_message(desiredQueue character varying(150)) 
RETURNS returned_message AS $$
    DECLARE result returned_message;
    BEGIN
    lock table queued_message in exclusive mode;
    update queued_message
    set 
        status='IN_PROGRESS'
    where
        id in (
        select
            id
        from
            queued_message
        where
            status='SUBMITTED' and queue=desiredQueue
        order by 
            id asc
        limit 1
        )
    returning * into result;
    RETURN result; 
END;$$LANGUAGE plpgsql; 


select * from get_next_message('BACKFILL_REQUEST')

Answer 1

You can create a function returning needed values. Each function executes as transaction.
Do not place “begin;” and “commit;” in the function body. Function below should work I believe.

create or replace function set_in_progress()
returns setof queued_message 
language sql as $$
    lock table queued_message in exclusive mode;
    update 
        queued_message
    set 
        status='IN_PROGRESS'
    where
        id in (
            select
                id
            from
                queued_message
            where
                status='SUBMITTED' and queue='BACKFILL_REQUEST'
            order by 
                id asc
            limit 1
        )
    returning *;
$$;

select * from set_in_progress();