问题
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.
回答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.
回答2:
Backtracking would require locking until completion. This would be bad, especially when 10 tables can all utilize the same sequence. If you want an order don't use a sequence use a window function like row_number().
来源：https://stackoverflow.com/questions/3081899/question-regarding-postgresql-sequences