Using DECODE on a parameter in WHERE clause will shortcircuit using the index? - oracle

Question

I have a stored procedure with multiple mandatory parameters and a SELECT statement inside it which has multiple conditions in its WHERE clause, like below:
SELECT *
FROM TABLE
WHERE column_1 = param_1
AND column_2 = param_2
AND column_3 = param_3;
This query works fine and it uses the indexes on the table correctly. But a change in requirements implied adjusting the procedure so that you can pass it less parameters, so maybe just the first two, but we want the procedure to work with minimal changes to the stored procedure.
One of the suggestions I've made was to use a DECODE function to treat each possibly NULL parameter, like this:
SELECT *
FROM TABLE
WHERE column_1 = param_1
AND column_2 = param_2
AND column_3 = DECODE(param_3, null, column_3);
And this way, I considered that because the function is not applied on the table column, the index will still be used. I have made some tests and the query still works and uses the indexes even in this situation.
But I'm still getting contradicted by our architect (with no other explanations), that the query will not use the index because I'm using a function in the WHERE clause.
I'm not sure if my change is enough proof that it will always use the index, or if there are other situations which I should check and in which the index might not be used because of the DECODE function.
Any help / suggestions / information will be very much appreciated.

Answer 1

You are right. Test it and prove it.
Setup
SQL> CREATE TABLE t AS SELECT LEVEL id FROM dual CONNECT BY LEVEL <=10;
Table created.
SQL>
SQL> CREATE INDEX id_indx ON t(ID);
Index created.
Test case
Normal query, without any function:
SQL> set autot on explain
SQL>
SQL> SELECT * FROM t WHERE ID = 5;
ID
----------
5
Execution Plan
----------------------------------------------------------
Plan hash value: 1629656632
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| ID_INDX | 1 | 3 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("ID"=5)
Using DECODE on the value(not on column):
SQL> SELECT * FROM t WHERE ID = decode(5, NULL, 3, 5);
ID
----------
5
Execution Plan
----------------------------------------------------------
Plan hash value: 1629656632
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| ID_INDX | 1 | 3 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("ID"=5)
Using NVL on the value(not on column):
SQL> SELECT * FROM t WHERE ID = nvl(5, 3);
ID
----------
5
Execution Plan
----------------------------------------------------------
Plan hash value: 1629656632
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| ID_INDX | 1 | 3 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("ID"=5)
Above all the three cases, index is used.
DECODE on the column:
SQL> SELECT * FROM t WHERE decode(ID, NULL, 3, 5) = 5;
ID
----------
1
2
3
4
5
6
7
8
9
10
10 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 3 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(DECODE(TO_CHAR("ID"),NULL,3,5)=5)
NVL on the column:
SQL> SELECT * FROM t WHERE nvl(ID, 3) = 3;
ID
----------
3
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 3 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(NVL("ID",3)=3)
SQL>
As expected, index is not used as you are applying a function on the column having a regular index. You need a function-based index.
So, you are right, you don't have to worry about index usage when you are not applying the function on the column, but on the parameter value.