thanks rsuri, yes only i create an index in case_reporter2site and

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: RULE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (BY ROWID) OF 'TABLE_CASE'
3 2 INDEX (RANGE SCAN) OF 'CASE_REPORTER2SITE'


Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
19 consistent gets
1 physical reads
0 redo size
132 bytes sent via SQL*Net to client
274 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed


only 1 physical reads, that's tuning

thanks