Index usage error

Question

i am having a table in my schema named provider_rate_history
PROVIDER_RATE_HISTORY_ID      NOT NULL      NUMBER
WORK_ORDER_HISTORY_ID      NOT NULL      NUMBER
PROVIDER_RATE_TYPE_ID      NOT NULL      NUMBER
CREATED_BY      NOT NULL      NUMBER
DATE_CREATED      NOT NULL      DATE
MODIFIED_BY           NUMBER
DATE_MODIFIED           DATE
I created an index named provider_rate_type_idx on the provider_rate_type_id column of the table, but when i am using the following query index is used, but if i replace count(*) keyword in the select statement with any of the column name of the table a ful table scan is performed. What could be the reason for this error and how to correct it
select count(*) from provider_rate_history
where rate_type_id=7;
Index is used, if we replace count(*) with any column name of the table index is not used
select work_order_history_id from provider_rate_history
where rate_type_id=7;
Index is not used

Answer 1

Hi,
Why count(*) will lead to full table scan?
APC have clearly told reason for not using Index.
Hope this illustration helps to clear the situation.
SQL> create table test111 as select * from all_objects where rownum < 1001;
Table created.
SQL> desc test111
Name Null? Type
OWNER NOT NULL VARCHAR2(30)
OBJECT_NAME NOT NULL VARCHAR2(30)
SUBOBJECT_NAME VARCHAR2(30)
OBJECT_ID NOT NULL NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(19)
CREATED NOT NULL DATE
LAST_DDL_TIME NOT NULL DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)
SQL> create index test111_indx1 on test111 (object_id);
Index created.
SQL>
SQL> set autotrace on
SQL> select count(1) from test111;
COUNT(1)
1000
Execution Plan
Plan hash value: 1326770390
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 1 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| TEST111_INDX1 | 1000 | 3 (0)| 00:00:01 |
Note
- dynamic sampling used for this statement
Statistics
5 recursive calls
0 db block gets
23 consistent gets
3 physical reads
0 redo size
206 bytes sent via SQL*Net to client
239 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
SQL> select count(distinct owner) from test111;
COUNT(DISTINCTOWNER)
3
Execution Plan
Plan hash value: 991123090
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 1 | 17 | 5 (0)| 00:00:01 |
| 1 | SORT GROUP BY | | 1 | 17 | | |
| 2 | TABLE ACCESS FULL| TEST111 | 1000 | 17000 | 5 (0)| 00:00:01 |
Note
- dynamic sampling used for this statement
Statistics
27 recursive calls
0 db block gets
33 consistent gets
0 physical reads
0 redo size
233 bytes sent via SQL*Net to client
239 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>