"Segmentation fault: 11" when trying to vacuum Apple Mail index Reading up on how to improve my suddenly-very-slow Mail app, I found the tip to rebuild the sqlite index:


  sqlite3 ~/Library/Mail/V4/MailData/Envelope\ Index vacuum;


But I get an immediate "Segmentation Fault: 11" and nothing happens. Can't figure out what that means or how to solve it. It's about the first time Google hasn't provided an answer for me. Anyone? (I'm on 10.12.x which is why it's V4.)
‘Segmentation fault: 11’ almost always means the process ran out of RAM whilst trying to perform some operation/calculation. This would corroborate with your slow Mail due to the size of the database you're trying to process (or it's just corrupted in such a way).

It may also be worth checking your free disk space, since the VACUUM command specifically requires plenty of space:


  The VACUUM command works by copying the contents of the database into a temporary database file and then overwriting the original with the contents of the temporary file. […] as much as twice the size of the original database file is required in free disk space.


https://sqlite.org/lang_vacuum.html