Result cache not working?

Question

Hi,
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
SQL> show parameter result;
NAME TYPE VALUE
client_result_cache_lag big integer 3000
client_result_cache_size big integer 0
result_cache_max_result integer 5
result_cache_max_size big integer 10M
result_cache_mode string MANUAL
result_cache_remote_expiration integer 0
The above are the initialization parameters set and the DB version.
Now i have created a table as
create table objects as select * from dba_objects;
then issued the following query.
select /* result_cache */ object_Type,count(*) from objects group by object_type;
it appears that the results of the above query is not cached in the result cache. using
set autotrace traceonly explain confirms that there is no result cache or the CACHE_ID in the explain plan. also the v$result_cache_statistics confirm the same.
But after this when i issued the following statement.
alter session set result_cache_mode=force;
the same below sql statement again
select /* result_cache */ object_Type,count(*) from objects group by object_type;
it appears that now the result cache is being used.
My question is despite using the result_cache hint and trying the above statement when the result_cache_mode=auto/manual. The result cache is not being used or the results of the SQL statements are not being cached at all despite using the result_cache hint and the table being not used at all. and if i use the result_cache_mode=force then the result cache is being used.
Can any one please explain this behavior. Cursor_sharing is set to EXACT. and i am issuing the same SQL statement without any changes at all.
Thanks,

Answer 1

There is a syntax problem with your queries:
select /* result_cache */.... is just a comment, not a hint. For creating a hint, you need "+": select /*+ result_cache */....