There are files such as db13507344071350313059_152-tmp in the defaultdb/db folder.
What are these files ending in tmp? When a search is performed, it seems the data that is in these files are not indexed. How do I tell splunk to index such files?
These are temporary files that are created by the online fsck process when Splunk detects a bad index and tries to restore the buckets for that index. They are essentially, a working copy to use for restoring the index until the index has been restored to a good state. Once the -tmp folder has restored all the data for the working copy, it will then replace the same named folder. This is to ensure that data is not lost while the restoration is running.
Sometimes, these -tmp folders can be left behind if the online fsck process exits ungracefully. Generally, it should be safe to delete the folder as it is only a copy of the same named db_* folder in the same directory. The online fsck tool will simply re-create the -tmp folder if it is needed again.