Reducing .local/share/akonadi

I use kmail to access three IMAP accounts, one POP3 account, and a local 
folder.

Since my .local/share/akonadi folder is pretty big (1.3 GB):

$ du -h -d 1 .local/share/akonadi
401M    .local/share/akonadi/db_data
497M    .local/share/akonadi/search_db
377M    .local/share/akonadi/file_db_data
4.0K    .local/share/akonadi/db_misc
1.3G    .local/share/akonadi

I was wondering whether there is a way to clear up some space. 

For instance, what's the purpose of the search_db directory?

Thanks & Regards