"lei q" vs mairix notes...

[Eric Wong <e@80x24.org>]

I'm not sure if this should be in the lei-q(1) manpage or
another manpage, probably another.  There ought to be a similar
doc for notmuch and any other existing mail things I'm not
familiar with.

This is intended to be a neutral document to help and set
expectations for mairix users should they attempt to use lei.
It is NOT intended as advocacy document.

mairix and "lei q" share some similarities around common search
prefixes ("f:", "s:", "nq:") but there are several differences
users familiar with mairix should be aware of.

- lei (Xapian) uses ".." for date and size ranges, mairix uses "-".
  This is due to how the Xapian query parser works.

- lei uses git(1) for date and time parsing; mairix has its own
  syntax documented in mairix(1).

- lei does not support MH, yet

- lei currently requires mail to be imported into git ("lei import");
  mairix indexes mail in IMAP, Maildir, MH, mbox directly
  lei may attempt to index mail outside of git if there's interest:
  https://public-inbox.org/meta/20210303035359.GA14438@dcvr/

- mairix can use symlinks and/or hardlinks to speed up writing
  results when using Maildirs; lei must always extract messages
  from git, which will always be slower.

- mairix has different rules around substring matches, negation,
  combining, etc. than Xapian <https://xapian.org/docs/queryparser.html>

- lei doesn't yet support config file entries for output
  (but will support saved searches)

- --raw-output and --excerpt-output in mairix aren't yet
  supported, but the default JSON output in "lei q" may be
  similar

- lei indexes positional data by default (and currently lacks a
  configuration knob in the CLI), so indices use significantly
  more space.

- lei is still in its infancy and far from complete

Again, this is intended to be a neutral document and not
advocacy.  Help appreciated with corrections and addendums.