Help needed for query plan change situations

Hello,

we have mariaDB version 5.5.37 .

We stumbled into an interesting situation:
– at some point in time the query plan has changed making the server behave poorly
– after a restart we noticed that the query plan was different behaving normally

I've asked our DBA's to try and use this: https://mariadb.com/kb/en/mariadb/how-to-force-query-plans/ in order to have some sort of control.

The query is fairly complex contains multiple joins and unions, this is the last entry in the explain result:
22.
id: NULL
select_type: UNION RESULT
table: <union2,3>
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra:

I would like some help for:
– getting info on why and how the optimizer made a choice (general optimizer info?)
– what is your experience dictates for this situations ?

Thank you .