Bacula is slow with mysql 5.7

[Jolan]

on Ubuntu 16.10, using :
bacula-common-mysql 7.4.3+dfsg-1+sid1build1
mysql-server-5.7 5.7.17-0ubuntu0.16.10.1

some requests are very slow, in particular those used for building the file list when restoring.
these requests usualy take less than 1sec. but are taking up to 5mn on the same tables.

this is due to the mysql 5.7 new optimizer_switch "derived_merge". setting it to off solves the issue.