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I need to have a custom unique identifier (sequence). In my table there is a field ready_to_fetch_id that will be null by default and when my message is ready to be delivered then i make it update with unique max id, this is quite heavy process as load increasing.
So it it possible to have some sequence in postgres that allow null and unique ids.
Allowing NULL values has nothing todo with sequences. If your column definition allows NULLs you can put NULL values in the column. When you update the column you take the nextval from the sequence.
Notice that if you plan to use the ids to keep track of which rows you have already processed that it won’t work perfectly. When two transactions are going to update the ready_to_fetch_id column simultaneous the transaction that started last might commit first which means that the higher id of the last transaction to start will become visible before the lower id the earlier transaction is using.