Job queue as SQL table with multiple consumers (PostgreSQL)

Question

I have a typical producer-consumer problem:

Multiple producer applications write job requests to a job-table on a PostgreSQL database.

The job requests have

Answer 1

Okay, here is the solution that is working for me, based on the link from jordani. As some of my problems were in the way Qt-SQL works, I've included the Qt code:

QSqlDatabase db = GetDatabase();
db.transaction();
QSqlQuery lockQuery(db);
bool lockResult = lockQuery.exec("LOCK TABLE serverjobrecord IN ACCESS EXCLUSIVE MODE; ");
QSqlQuery query(db);
query.prepare(    
"UPDATE jobrecord "
"  SET \"owner\"= :owner, state = :reserved "
"  WHERE id = ( "
"    SELECT id from jobrecord WHERE state = :queued ORDER BY id LIMIT 1 "
"  ) RETURNING id;"
);
query.bindValue(":owner", pid);
query.bindValue(":reserved", JobRESERVED);
query.bindValue(":queued", JobQUEUED); 
bool result = query.exec();

To check, if multiple consumers process the same job, I added a rule and a log-table:

CREATE TABLE serverjobrecord_log
(
  serverjobrecord_id integer,
  oldowner text,
  newowner text
) WITH ( OIDS=FALSE );


CREATE OR REPLACE RULE ownerrule AS ON UPDATE TO jobrecord
WHERE old.owner IS NOT NULL AND new.state = 1 
DO INSERT INTO jobrecord_log     (id, oldowner, newowner) 
    VALUES (new.id, old.owner, new.owner);

Without the LOCK TABLE serverjobrecord IN ACCESS EXCLUSIVE MODE; statement, the log-table fills occasionaly with entries, were one consumers has overwritten the values of another, but using the LOCK statement, the log-table remains empty :-)