Optimizer chooses slow parallel plan in Oracle 9.2

Question

We have an application that we are migrating from Oracle8 (8.1.7) to Oracle9 (9.2.0). We've changed from DMTs to LMTs too. We've come across a performance issue when running a application benchmark that we've isolated down to a simple 1 row look up query in a rather small table, initially 8 rows growing to several thousand. The table is 2 columns, an int ID value and a VARCHAR2(254) column. There is a unique index on the name column. The prepared query is of the form:
select id from table_name where name = <bind_variable>;
In Oracle8 the CBO chooses a serial plan of unique index lookup followed by table access by rowid to get the result from the table row. Oracle9 chooses a parallel table scan of degree default (effective degree 4) for the same query even when the table only has 8 rows.
Over thousands of executions the Oracle8 query average 2-3 msec each. However, the exactly the same data and same sequence of queries the Oracle9 query averages 150 msec each. Clearly the cost of setup and tear down of executing a parallel query here is too expensive.
My question is why is the Oracle9 optimizer making such a different, and in my opinion poorer, choice of query plan than the Oracle8 optimizer?
I should note the optimizer stats are the same and result from exactly the same DBMS_STATS.GATHER_TABLE_STATS calls and Oracle9 is behaving exactly the same with the OPTIMIZER_FEATURES_ENABLE set to 9.2.0 and 8.1.7.

Answer 1

Gabriele,
As indicated in the original post - the following work-around might work for you for bug 9743250. What this does is essentially force the optimizer to ALWAYS use the spatial index. That is usually good - unless the query is actually more efficient not using the index, such as when it is much cheaper to just to a FTS. Anyhow this is what you can do from 11.2.0.1 to 11.2.0.3 - fixed in 11.2.0.4 and 12.1.0.1:
connect /as sysdba
alter session set current_schema=MDSYS;
DISASSOCIATE STATISTICS FROM INDEXTYPES spatial_index FORCE
DISASSOCIATE STATISTICS FROM PACKAGES sdo_3gl FORCE;
DISASSOCIATE STATISTICS FROM PACKAGES prvt_idx FORCE;
Bryan