在iOS中使用SQLite PRAGMA同步= OFF和journal_mode = MEMORY进行更新的最坏情况行为

Question

By using both the synchronous=OFF and journal_mode=MEMORY options, I am able to reduce the speed of updates from 15 ms to around 2 ms which is a major performance improvement. These updates happen one at a time, so many other optimizations (like using transactions about a bunch of them) are not applicable.

According to the SQLite documentation, the DB can go 'corrupt' in the worst case if there is a power outage of some type. However, is not the worst thing that can happen is for the data to be lost, or possibly part of a transaction to be lost (which I guess is a form of corruption). Is it really possible for arbitrary corruption to occur with either of these options? If so, why?

I am not using any transactions, so partially written data from transactions is not a concern, and I can handle loosing data once in a blue moon. But if 'corruption' means that all the data in the DB can be randomly changed in an unpredictable way, that would be a strong reason to not use these options.

Does any one know what the real worst-case behavior would be on iOS?

Answer 1

Tables are organized as B-trees with the rowid as the key. If some writes get lost while SQLite is updating the tree structure, the entire table might become unreadable. (The same can happen with indexes, but those could be simply dropped and recreated.)

Data is organized in pages (typically 1 KB or 4 KB). If some page update gets lost while some tree is being reorganized, all the data in these pages (ie, some random rows from the table with nearby rowid values) might become corruped.

If SQLite needs to allocate a new page, and that page contains plausible data (eg, deleted data from the same table), and the writing of that page gets lost, then you have incorrect data in the table, without the ability to detect it.