Question

In Oracle (10g), when I use a View (not Materialized View), does Oracle take into account the where clause when it executes the view?

Let’s say I have:

MY_VIEW =
SELECT * 
FROM PERSON P, ORDERS O
WHERE P.P_ID = O.P_ID

And I then execute the following:

SELECT * 
FROM MY_VIEW
WHERE MY_VIEW.P_ID = '1234'

When this executes, does oracle first execute the query for the view and THEN filter it based on my where clause (where MY_VIEW.P_ID = ‘1234’) or does it do this filtering as part of the execution of the view? If it does not do the latter, and P_ID had an index, would I also lose out on the indexing capability since Oracle would be executing my query against the view which doesn’t have the index rather than the base table which has the index?

Answer 1

It will not execute the query first. If you have a index on P_ID, it will be used.

Execution plan is the same as if you would merge both view-code and WHERE-clause into a single select statement.

You can try this for yourself:

EXPLAIN PLAN FOR
SELECT * 
FROM MY_VIEW
WHERE MY_VIEW.P_ID = '1234'

followed by

SELECT * FROM TABLE( dbms_xplan.display );

---------------------------------------------------------------------------------
|Id | Operation                    | Name   |Rows| Bytes | Cost (%CPU)| Time    |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT             |        |  1 |    52 |     2   (0)| 00:00:01|
| 1 |  NESTED LOOPS                |        |  1 |    52 |     2   (0)| 00:00:01|
| 2 |   TABLE ACCESS BY INDEX ROWID| PERSON |  1 |    26 |     2   (0)| 00:00:01|
| 3 |    INDEX UNIQUE SCAN         | PK_P   |  1 |       |     1   (0)| 00:00:01|
| 4 |   TABLE ACCESS BY INDEX ROWID| ORDERS |  1 |    26 |     0   (0)| 00:00:01|
| 5 |    INDEX RANGE SCAN          | IDX_O  |  1 |       |     0   (0)| 00:00:01|
---------------------------------------------------------------------------------