Question

I have a question regarding postgresql sequences.

For instance, for bigserial datatype, is it true that the sequence is advanced, then the number is retrieved and even if the insertion/committing is not successful, the sequence doesn’t backtracks. Which means the next time I might be doing insertion to the table, that might be a gap in the sequence number.

Theres a before insert row trigger on my table and Im using psycopg2.

thanks in advance.

Answer 1

even if the insertion/committing is
not successful, the sequence doesn’t
backtracks. Which means the next time I might be doing insertion to the table, that might be a gap in the sequence number.

Yes, that’s true, And that’s fine.
One usually wants a sequence to get values in a table that are unique (typically for a PK)
and gaps don’t matter at all.

If you are curious: this is natural behaviour if one thinks about concurrency. Suppose a transaction T1 inserts a row, getting a PK1 from a sequence, uses that value to build another records in other tables… in the meantime (before T1 commits) another transaction T2 inserts a row in the same table. Then T1 rollbacks and T2 commits…

BTW: If you want a “gap-less” sequence … first ask yourself if you really want that (usually you really don’t – and requiring that frequently points to a conceptual problem in your design)… but if you really need it, you can read this.