INDEX UNIQUE SCAN instead of INDEX FULL SCAN or TABLE ACCESS FULL

Question

I have calculated statistics in all tables and indexes
I have a table and a view and when I put it
SELECT *
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID          
WHERE (B.FK_ID_XXX = 1)
If I see the execution plan:
In TABLE_A make a
TABLE ACCESS BY INDEX ROWID
INDEX UNIQUE SCAN (FIELD_A_TABLE_A_PK)
It’s OK. I NEED IT (INDEX UNIQUE SCAN)
But If I put
SELECT A.Field_1, A.Field_2, A.Field_3, A.Field_4
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID          
WHERE (B.FK_ID_XXX = 1)
In table A make a TABLE ACCESS FULL.
Then If I put:
SELECT /*+ INDEX(A FIELD_A_TABLE_A_PK) */ A.Field_1, A.Field_2, A.Field_3, A.Field_4
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID          
WHERE (B.FK_ID_XXX = 1)
If I see the execution plan:
In TABLE_A make a
TABLE ACCESS BY INDEX ROWID
INDEX UNIQUE SCAN (FIELD_A_TABLE_A_PK)
It’s OK. I NEED IT (INDEX UNIQUE SCAN)
Finally, If I put other tables and views in the query (I NEED IT)
For example:
SELECT /*+ INDEX(A FIELD_A_TABLE_A_PK) */ A.Field_1, A.Field_2, A.Field_3, A.Field_4
FROM TABLE_A A
INNER JOIN VIEW_B B ON A.KEY_ID = B.PFK_KEY_ID          
INNER JOIN TABLE_C….
LEFT JOIN VIEW_D….
WHERE (B.FK_ID_XXX = 1)
If I see the execution plan:
In TABLE_A make a
TABLE ACCESS BY INDEX ROWID
INDEX FULL SCAN (FIELD_A_TABLE_A_PK)
I need INDEX UNIQUE SCAN instead of INDEX FULL SCAN or TABLE ACCESS FULL.
How can obtain it?
What happens???
Thanks!

Answer 1

Notice the difference in cardinality between your two select statements:
SELECT STATEMENT, GOAL = ALL_ROWS Cost=5 Cardinality=1
SELECT STATEMENT, GOAL = ALL_ROWS Cost=10450 Cardinality=472161Apparently since the optimizer believed the first statement was going to return one row, it used an index. But in the second statement it believed it was going to return nearly the whole table (didn't you say it had around 500k rows?). Hence full table scan.