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Tom Lane authored
In the previous coding, we simply issued ALTER SEQUENCE RESTART commands, which do not roll back on error. This meant that an error between truncating and committing left the sequences out of sync with the table contents, with potentially bad consequences as were noted in a Warning on the TRUNCATE man page. To fix, create a new storage file (relfilenode) for a sequence that is to be reset due to RESTART IDENTITY. If the transaction aborts, we'll automatically revert to the old storage file. This acts just like a rewriting ALTER TABLE operation. A penalty is that we have to take exclusive lock on the sequence, but since we've already got exclusive lock on its owning table, that seems unlikely to be much of a problem. The interaction of this with usual nontransactional behaviors of sequence operations is a bit weird, but it's hard to see what would be completely consistent. Our choice is to discard cached-but-unissued sequence values both when the RESTART is executed, and at rollback if any; but to not touch the currval() state either time. In passing, move the sequence reset operations to happen before not after any AFTER TRUNCATE triggers are fired. The previous ordering was not logically sensible, but was forced by the need to minimize inconsistency if the triggers caused an error. Transactional rollback is a much better solution to that. Patch by Steve Singer, rather heavily adjusted by me.
Tom Lane authoredIn the previous coding, we simply issued ALTER SEQUENCE RESTART commands, which do not roll back on error. This meant that an error between truncating and committing left the sequences out of sync with the table contents, with potentially bad consequences as were noted in a Warning on the TRUNCATE man page. To fix, create a new storage file (relfilenode) for a sequence that is to be reset due to RESTART IDENTITY. If the transaction aborts, we'll automatically revert to the old storage file. This acts just like a rewriting ALTER TABLE operation. A penalty is that we have to take exclusive lock on the sequence, but since we've already got exclusive lock on its owning table, that seems unlikely to be much of a problem. The interaction of this with usual nontransactional behaviors of sequence operations is a bit weird, but it's hard to see what would be completely consistent. Our choice is to discard cached-but-unissued sequence values both when the RESTART is executed, and at rollback if any; but to not touch the currval() state either time. In passing, move the sequence reset operations to happen before not after any AFTER TRUNCATE triggers are fired. The previous ordering was not logically sensible, but was forced by the need to minimize inconsistency if the triggers caused an error. Transactional rollback is a much better solution to that. Patch by Steve Singer, rather heavily adjusted by me.
