set optimizer

Hello , (google translate)

I am a little fun with the optimizer .

sql>s elec t count (distinct idvisiteur ) from src_inbox__at__lm__visites ; + ---------- + | L1 | + + ========== | 75528080 | + ---------- + 1 tuple ( 3m 51s )

sql> set optimizer = ' minimal_pipe ;

sql> select count (distinct idvisiteur ) from src_inbox__at__lm__visites ; + ---------- + | L1 | + + ========== | 75528080 | + ---------- + 1 tuple (1m 28s )

sql> set optimizer = ' default_pipe ;

sql> select count (distinct idvisiteur ) from src_inbox__at__lm__visites ; + ---------- + | L1 | + + ========== | 75528080 | + ---------- + 1 tuple ( 3m 54s )

It's very interesting . Is there a risk to change optimizer ? What are the best practices ?

thank you