Oracle Database Discussions

I have a problem with an Oracle 11g Release 11.2.0.1.0 database, where a query such as:
SELECT fields
FROM all_constraints ac,
all_cons_columns loc,
all_cons_columns rem
WHERE ac.table_name = '&tab'
AND ac.constraint_type = 'R'
AND ac.owner = '&own'
AND ac.owner = loc.owner
AND ac.constraint_name = loc.constraint_name
AND ac.r_owner = rem.owner
AND ac.r_constraint_name = rem.constraint_name
AND loc.position=rem.position
ORDER BY constraint_name, loc_pos;

Takes a very very long time, but adding the /*+ RULE */ hint makes it execute quite fast.

When I ran:
SQL> exec DBMS_STATS.DELETE_DICTIONARY_STATS();

PL/SQL procedure successfully completed.

SQL> exec DBMS_STATS.DELETE_FIXED_OBJECTS_STATS();

PL/SQL procedure successfully completed.

Then afterwards, the query was fast (without the /*+ RULE */ hint).

Gathering the stats again:
SQL> exec DBMS_STATS.GATHER_DICTIONARY_STATS();

Slowed it back down.

I can't imagine that best practice is to DELETE_DICTIONARY_STATS()? Can anyone explain what is going on and advise?

Many thanks for your time!