Imagine that you have a postgresql server that is capable of handling 10 SQL queries over psql (Network TCP ) at once, without service degrading. ie the performance of postgresql does not slow down, and each query is fulfilled within an expected and acceptable time.
Imagine that as the number of concurrent SQL queries increases over and above 10, that the performance degrades and each query takes longer to fulfil. Eventually the queries take an excessive amount of time that is not acceptable for good service delivery. Often if we could simply queue the SQL queries (psql sessions) and allow only say 10 concurrent queries active at any given time, performance overal will be better.Mas:

http://mike-k-linux-mumbling.blogspot.com/2012/01/postgresql-and-haproxy-queuing.html