Thinking aloud...

On Thu, Aug 10, 2017 at 3:05 PM, marcelo <> 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

​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 +

David J.

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