Index ignored in query??

Question

This is probably a basic question about indexes, thanks for your insights.
I have a table VOUCHER with some columns including a TICKETNO column for which there is a primary key index, defined as follows:
CREATE UNIQUE INDEX VOUCHER_PK ON VOUCHER (TICKETNO)
The table contain approximately 20 million records.
We are using Oracle 9i RAC on HP-UX 11.11
I am running the following query, which runs for 20 minutes and then fails:
Oracle9i Enterprise Edition Release 9.2.0.8.0 - 64bit Production
With the Partitioning and Real Application Clusters options
JServer Release 9.2.0.8.0 - Production
SQL> select count(*) from VOUCHER
2 where TICKETNO between 100000 and 100100
3
SQL> /
select count(*) from VOUCHER
ERROR at line 1:
ORA-01555: snapshot too old: rollback segment number 741 with name
"_SYSSMU741$" too small
I am confused... Why counting 100 records with an indexed column takes so long and fail?

Answer 1

Thanks for the feedback. I am going to run the ANALYZE suggestion tonight.
Here are the results of the queries asked for:
SELECT Table_Name, Status FROM User_Indexes WHERE Index_Name = 'VOUCHER_PK';
OWNER TABLE_NAME STATUS
SMVDBA VOUCHER VALID
SELECT Column_Name FROM User_Ind_Columns WHERE Index_Name = 'VOUCHER_PK';
COLUMN_NAME
TICKETNO
SELECT Last_Analyzed FROM User_Indexes WHERE Index_Name = 'VOUCHER_PK';
LAST_ANAL
SELECT Last_Analyzed FROM User_Tables WHERE Table_Name = 'VOUCHER';
LAST_ANAL
21-FEB-08
EXPLAIN PLAN FOR
select count(*) from VOUCHER
where TICKETNO between 100000 and 100100;
Explained
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY('', '', 'ALL'));
PLAN_TABLE_OUTPUT
| Id | Operation | Name | Rows | Bytes | Cost |
| 0 | SELECT STATEMENT | | | | |
| 1 | SORT AGGREGATE | | | | |
| 2 | TABLE ACCESS FULL | VOUCHER | | | |
Note: rule based optimization, PLAN_TABLE' is old version
10 rows selected.
EXPLAIN PLAN FOR
select /*+ INDEX(V VOUCHER_PK) */ count(*) from VOUCHER V
where TICKETNO between 100000 and 100100;
Explained.
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY('', '', 'ALL'));
PLAN_TABLE_OUTPUT
| Id | Operation | Name | Rows | Bytes | Cost |
| 0 | SELECT STATEMENT | | 1 | 18 | 5 |
| 1 | SORT AGGREGATE | | 1 | 18 | |
| 2 | INDEX FAST FULL SCAN| VOUCHER_PK | 83788 | 1472K| 5 |
Note: cpu costing is off, PLAN_TABLE' is old version
10 rows selected.