Thinking aloud... On Thu, Aug 10, 2017 at 3:05 PM, marcelo <marcelo.nico...@gmail.com> wrote:
> In some table, I have a bigint column which at the app level can be null. > Call it "DocumentNumber", and of course is not the PK. > In most cases, the applications give some value to the column. > > But sometimes, the value remains null, expecting the backend or someone > assign it a unique value. > > Could I use a sequence only when the field arrives to the backend as null? > How? Using a triger? > Can you reserve a portion of the value range for auto-generated numbers that application-assigned values will take on? If so it would reasonably simple to invoke nextval() in a trigger. How could I get the max value for the column and increment it by one, but > with concurrency warranty? Something as a table lock? > One option would be to maintain the value in a separate table that you update on insert using "UPDATE tbl SET col = col + 1 RETURNING col INTO new_doc_num" You could probably make it an unlogged table as well and you'd return from the trigger function with new_doc_num if its non-null otherwise you'd branch and re-create the record before returning the just queried maximum + 1. David J.