Result cache invalidated

Question

Hi,
Oracle version 11.2.0.1 on Linux
I understand thatt result cache (RC) invalidation is at table level.
I did a simple test:
create table customer (custno number, custname varchar2(30));
Table created.
insert into customer (custno,custname) values (1,'Customer_1');
insert INTO CUSTOMER (custno,custname) values (2,'Customer_X');
select * from customer;
    CUSTNO CUSTNAME
         1 Customer_1
         2 Customer_X
commit;
Commit complete.Now I invoke result cache here
select /*+ RESULT_CACHE */ * FROM customer where custno=1;
Execution Plan
Plan hash value: 2844954298
| Id  | Operation          | Name                       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |                            |     1 |    30 |     3   (0)| 00:00:01 |
|   1 |  RESULT CACHE      | ggb2vz6jcvcn5ajzqh406j3n85 |       |       |            |          |
|*  2 |   TABLE ACCESS FULL| CUSTOMER                   |     1 |    30 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - filter("CUSTNO"=1)
Result Cache Information (identified by operation id):
   1 - column-count=2; dependencies=(SCRATCHPAD.CUSTOMER); name="select /*+ RESULT_CACHE */ * FROM customer where custno=1"Invoke RC for second row query
select /*+ RESULT_CACHE */ * FROM customer where custno=2;
Execution Plan
Plan hash value: 2844954298
| Id  | Operation          | Name                       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |                            |     1 |    30 |     3   (0)| 00:00:01 |
|   1 |  RESULT CACHE      | fc8t6svvz6whh0gc8vcaxrh668 |       |       |            |          |
|*  2 |   TABLE ACCESS FULL| CUSTOMER                   |     1 |    30 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - filter("CUSTNO"=2)
Result Cache Information (identified by operation id):
   1 - column-count=2; dependencies=(SCRATCHPAD.CUSTOMER); name="select /*+ RESULT_CACHE */ * FROM customer where custno=2"OK they are stored as separate result cache
Now update the second row in that table in another session
update customer set custname ='Customer_2' where custno=2;
1 row updated.
commit;
Commit complete.Now query custno=2 from the first session
select /*+ RESULT_CACHE */ * FROM customer where custno=2;
Execution Plan
Plan hash value: 2844954298
| Id  | Operation          | Name                       | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |                            |     1 |    30 |     3   (0)| 00:00:01 |
|   1 |  RESULT CACHE      | fc8t6svvz6whh0gc8vcaxrh668 |       |       |            |          |
|*  2 |   TABLE ACCESS FULL| CUSTOMER                   |     1 |    30 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - filter("CUSTNO"=2)
Result Cache Information (identified by operation id):
   1 - column-count=2; dependencies=(SCRATCHPAD.CUSTOMER); name="select /*+ RESULT_CACHE */ * FROM customer where custno=2"The same result cache reference is still there. So does this mean that result cache is NOT invalidated despite the row being updated or I am doing something wrong here?
Thanks
Edited by: 902986 on 12-Feb-2012 13:26

Answer 1

The result cache id is a hash value for the query so that Oracle can later tell if a query will produce a result set that is already in the cache.
When you updated the table the result set in the cache was marked invalid. Then you ran the same query for record 2 and Oracle created a hash value for the query and the hash value is the same as the first time since the query is the same; that is, the query itself hashes to the same value. But the result cache contents for that query have changed and replace the old invalid contents.
If you query the result set cache you will get the new value not the old one since the old result set for your second query isn't there anymore.