Avoid Full Table Scan with subquery or analytic function in view - sql

Question

I can reproduce the following behavior both with Oracle 11 (see SQL Fiddle) and Oracle 12.
CREATE TYPE my_tab IS TABLE OF NUMBER(3);
CREATE TABLE test AS SELECT ROWNUM AS id FROM dual CONNECT BY ROWNUM <= 1000;
CREATE UNIQUE INDEX idx_test ON test( id );
CREATE VIEW my_view AS
SELECT id, COUNT(1) OVER ( PARTITION BY id ) AS cnt
FROM test;
The following case uses the index as expected:
SELECT * FROM my_view
WHERE id IN ( 1, 2 );
---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 52 | 2 (0)| 00:00:01 |
| 1 | VIEW | MY_VIEW | 2 | 52 | 2 (0)| 00:00:01 |
| 2 | WINDOW BUFFER | | 2 | 8 | 2 (0)| 00:00:01 |
| 3 | INLIST ITERATOR | | | | | |
|* 4 | INDEX UNIQUE SCAN| IDX_TEST | 2 | 8 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------------
The following case does not use the index even though the cardinality hint is provided:
SELECT * FROM my_view
WHERE id IN ( SELECT /*+ CARDINALITY( tab 2 ) */ COLUMN_VALUE
FROM TABLE( NEW my_tab( 1, 2 ) ) tab );
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 28 | 33 (4)| 00:00:01 |
|* 1 | HASH JOIN RIGHT SEMI | | 1 | 28 | 33 (4)| 00:00:01 |
| 2 | COLLECTION ITERATOR CONSTRUCTOR FETCH| | 2 | 4 | 29 (0)| 00:00:01 |
| 3 | VIEW | MY_VIEW | 1000 | 26000 | 4 (25)| 00:00:01 |
| 4 | WINDOW SORT | | 1000 | 4000 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | TEST | 1000 | 4000 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------
Edit:
Using an inline view and a JOIN instead of IN uses a similar plan:
SELECT /*+ CARDINALITY( tab, 2 ) */ *
FROM ( SELECT id, COUNT(1) OVER ( PARTITION BY id ) AS cnt FROM test ) t
JOIN TABLE( NEW my_tab( 1, 2 ) ) tab ON ( tab.COLUMN_VALUE = t.id );
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 56 | 33 (4)| 00:00:01 |
|* 1 | HASH JOIN | | 2 | 56 | 33 (4)| 00:00:01 |
| 2 | COLLECTION ITERATOR CONSTRUCTOR FETCH| | 2 | 4 | 29 (0)| 00:00:01 |
| 3 | VIEW | | 1000 | 26000 | 4 (25)| 00:00:01 |
| 4 | WINDOW SORT | | 1000 | 4000 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | TEST | 1000 | 4000 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Replacing the analytic function by a LEFT JOIN with GROUP BY does not help either:
SELECT *
FROM ( SELECT t.id, s.cnt
FROM test t
LEFT JOIN ( SELECT id, COUNT(*) AS cnt
FROM test
GROUP BY id
) s ON ( s.id = t.id )
)
WHERE id IN ( SELECT /*+ CARDINALITY( tab 2 ) */ COLUMN_VALUE
FROM TABLE( NEW my_tab( 1, 2 ) ) tab );
-----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 64 | 34 (6)| 00:00:01 |
|* 1 | HASH JOIN OUTER | | 2 | 64 | 34 (6)| 00:00:01 |
| 2 | NESTED LOOPS | | 2 | 12 | 30 (4)| 00:00:01 |
| 3 | SORT UNIQUE | | 2 | 4 | 29 (0)| 00:00:01 |
| 4 | COLLECTION ITERATOR CONSTRUCTOR FETCH| | 2 | 4 | 29 (0)| 00:00:01 |
|* 5 | INDEX UNIQUE SCAN | IDX_TEST | 1 | 4 | 0 (0)| 00:00:01 |
| 6 | VIEW | | 1000 | 26000 | 4 (25)| 00:00:01 |
| 7 | HASH GROUP BY | | 1000 | 4000 | 4 (25)| 00:00:01 |
| 8 | TABLE ACCESS FULL | TEST | 1000 | 4000 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------
Replacing the PL/SQL Collection by a subselect does not seem to help either. The CARDINALITY hint is considered (the plan says 2 rows), but the index is still ignored.
SELECT *
FROM ( SELECT id, cnt FROM my_view )
WHERE id IN ( SELECT /*+ CARDINALITY( tab 2 ) */ id FROM test tab );
---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 60 | 4 (25)| 00:00:01 |
| 1 | NESTED LOOPS | | 2 | 60 | 4 (25)| 00:00:01 |
| 2 | VIEW | MY_VIEW | 1000 | 26000 | 4 (25)| 00:00:01 |
| 3 | WINDOW SORT | | 1000 | 4000 | 4 (25)| 00:00:01 |
| 4 | TABLE ACCESS FULL| TEST | 1000 | 4000 | 3 (0)| 00:00:01 |
|* 5 | INDEX UNIQUE SCAN | IDX_TEST | 1 | 4 | 0 (0)| 00:00:01 |
---------------------------------------------------------------------------------
Adding WHERE tab.id <= 2 to the in-list-subquery uses the index, so the optimizer seems to "not take the CARDINALITY hint serious enough" when selecting from a view with analytic functions (or another subselect) and filtering by a list of values.
How can I make these queries use the index as expected?

Answer 1

I think the one problem might be that the optimizer refuses to merge a view (and consider any indexes on the underlying tables) when the outer query block contains PL/SQL functions (e.g. TABLE()).
If you manually expand the view and query the table directly, it can access the index fine:
SELECT id, COUNT(1) OVER ( PARTITION BY id ) AS cnt
FROM test
WHERE id IN ( SELECT COLUMN_VALUE
FROM TABLE( NEW my_tab( 1, 2 ) ) tab )
;
----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 6 | 31 (4)| 00:00:01 |
| 1 | WINDOW SORT | | 1 | 6 | 31 (4)| 00:00:01 |
|* 2 | HASH JOIN SEMI | | 1 | 6 | 30 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | IDX_TEST | 1000 | 4000 | 1 (0)| 00:00:01 |
| 4 | COLLECTION ITERATOR CONSTRUCTOR FETCH| | 8168 | 16336 | 29 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------
I'm not sure if there's a way to override this behavior, or if it's a limitation in the optimizer. I tried moving the TABLE function to a CTE, but that doesn't seem to help.