On 2/28/2016 9:19 PM, Rowan Worth wrote: > On 27 February 2016 at 00:02, Igor Tandetnik <igor at tandetnik.org> wrote: > >> On 2/26/2016 4:01 AM, Rowan Worth wrote: >> >>> In principle this is correct, but actually the database *file* is not >>> immediately modified in rollback mode. Instead when a page is modified the >>> original contents are saved to the rollback journal, and the page is >>> updated *in memory*. >>> >> >> ... until such time as the cache needs to be spilled - then it's updated >> in the database file. In fact, I'm pretty sure the rollback journal is not >> created as long as all the changes are entirely in RAM. >> > > The rollback journal is always created before the main database file is > modified. This must happen for durability's sake - if the changes were only > held in RAM then sqlite would have no way to recover a partially modified > database caused by a power failure mid-transaction.
By definition, as long as "all the changes are entirely in RAM", the main database file has not yet been modified. After all, if it has been modified, then some changes are already outside of RAM. In light of this rather obvious observation, I'm not quite sure what point you were trying to make with this paragraph. > > I simplified to make the main point stand out: it is not true that the >> database file remains "pristine" while the transaction is in progress, and >> changes are written only to the journal file. Instead, to the first >> approximation, the opposite is true. > > I agree it is not true in general, but it is mostly accurate for small > transactions No, it's never true (at least, not with the rollback journal - WAL is different still). Changes are not written to the journal; original unmodified pages are. -- Igor Tandetnik
