Thanks!

Actually, adding 0 in the select didn't help, but when I put it in the
where clause (WHERE date_fiche + 0 < :date_fiche ...), it works! It's
amazing how not using an index can make things go so much faster (in
this specific case), even though, for other queries, the same index
makes things go a lot faster...

Do you have any idea if there's a better way to formulate this type of
query?

Thanks a lot!
Jonathan Neve.