On 18 Oct 2012, at 2:32pm, Daniel Polski <dan...@agelektronik.se> wrote:
> I logically do understand that there can't be 2 writers updating the database > at the same time, but I don't understand why the second insert statement in > the example below won't work without finalizing the SELECT query? The fact that changes are initially made to a journal file (maybe a WAL file) and only moved to the normal database file later is a red herring. Similarly, where in the file changes are made (whether they modify existing pages or create new ones) is a red herring. You should consider all the files that make up a database to be one consistent whole. A SELECT needs to keep the database locked against changes. Because the results of the SELECT must reflect the data as it was when the command was executed. So until the SELECT is finalized (or has produced an error), no changes can be made to the data because that might make the results of the SELECT inconsistent. So you can run multiple SELECTs at one time, since there's no way for one SELECT to invalidate the data returned by another. But as soon as someone tries a command that would change the file, it has to be blocked, because all SELECTs under way must be allowed to finish before anything is allowed to make changes. So once again, consider all the steps involved in a statement: _prepare(), _step(), and _finalize() to be part of the same process, and once you've started it, finish it as quickly as possible. Earlier versions didn't even split this up into three statements: they provided just _exec() which does all three. It might be best to continue thinking of SQLite in this way. Simon. _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
