Optimize double max queries, index plan not used.

Question

Hi,
In my 9.2.0.8 I've got test case like :
create table t (id1 number ,id2 number) ;
begin
for i in 1 .. 1000000
loop
     insert into t values(i, i+10);
end loop;
end;
create index id1 on t(id1) ;
create index id2 on t(id2);
exec dbms_stats.gather_table_stats(user, 'T', cascade => true);
set autotrace traceonly
select max(id1), max(id2) from t;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=127 Card=1 Bytes=9)
   1    0   SORT (AGGREGATE)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=127 Card=1000000 Bytes=9000000)
Statistics
          0  recursive calls
          0  db block gets
       2068  consistent gets
          0  physical reads
          0  redo size
        232  bytes sent via SQL*Net to client
        238  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
whereas
SQL> select max(id1) from t;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=127 Card=1 Bytes=5)
   1    0   SORT (AGGREGATE)
   2    1     INDEX (FULL SCAN (MIN/MAX)) OF 'ID1' (NON-UNIQUE) (Cost=127 Card=1000000 Bytes=5000000)
Statistics
          0  recursive calls
          0  db block gets
          3  consistent gets
          1  physical reads
          0  redo size
        211  bytes sent via SQL*Net to client
        238  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> select max(id2) from t;
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=127 Card=1 Bytes=5)
   1    0   SORT (AGGREGATE)
   2    1     INDEX (FULL SCAN (MIN/MAX)) OF 'ID2' (NON-UNIQUE) (Cost=127 Card=1000000 Bytes=5000000)
Statistics
          0  recursive calls
          0  db block gets
          3  consistent gets
          1  physical reads
          0  redo size
        214  bytes sent via SQL*Net to client
        238  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedSo, why Oracle cant use both indexes to do optimal index min/max scan ?
How to rewrite that query ?
Regards
GregG

Answer 1

Hi,
Does this help,
with
p
as
(select max(id1)
from t),
q as
(select max(id2)
from t )
select * from p,qRegards,
CKLP