android - What are the .db-shm and .db-wal extensions in Sqlite databases?

Question

I am seeing some strange behavior with my application and the state of its database file after running some tests that close the database, delete it, and replace it with a test fixture. When I examine the database file with a tool on my debugging PC, it doesn't match what the application itself seems to be reporting. It's possible that this strange behavior is related to this bug.

I noticed that there are two files with the same base name as the database (with the normal .db extension.) The file extensions are .db-shm and .db-wal, and each is newer than the .db file's timestamp.

I assume that these are some type of temporary files. However, I am wondering if the application is terminated, shouldn't they be deleted? More importantly, I assume whatever data is stored in them is updated inside the .db file before the application is terminated by the operating system. Is this correct?

Answer 1

You are correct, these are temporary files created by SQLite. If you are manually deleting the main db you should probably delete these too. From what I can gather the WAL is a replacement for the rollback journal that enables SQLite to rollback changes when a transaction fails. How SQLite uses them and why they are kept around for so long is up to the authors of SQLite but in general SQLite seems pretty rock solid so I wouldn't worry too much about them. For more info take a look here:

http://www.sqlite.org/fileformat2.html#walindexformat

These files are a new feature of SQLite 3.7. I'm not sure if their existence relates to the bug you point out but the bug report suggests a work-around anyway.

UPDATE:

Better documentation about the WAL is here:

https://www.sqlite.org/wal.html

The contents of the WAL are periodically moved to the DB file but this is not guaranteed to occur each time the process exits. Thus when WAL is enabled each SQLite DB consists of two files on disk that must be preserved, both the .db file and the .db-wal file.

The .db-shm file is a shared memory file that contains only temporary data.