Limit rows examined in Oracle - oracle

Question

My table has millions of records. In this query below, can I make Oracle 12c examine the first X rows only instead of doing a full table scan?
The value of X, I imagine should be Offset + Fetch Next , so in this case 15
SELECT * FROM table OFFSET 5 ROWS FETCH NEXT 10 ROWS ONLY;
Thanks in advance
Edit 1
These are the tables involved and this is the actual query
Orders - This table has 113k records in my test DB ( and over 8 million in prod db like my original question mentioned)
--------------------------
| Id | SKUField1|SKUField2|
--------------------------
| 1 | Value1 | Value2 |
| 2 | Value2 | Value2 |
| 3 | Value1 | Value3 |
--------------------------
Products - This table has 2 million records in my test DB ( prod db is similar)
---------------
| PId| SKU_NUM|
---------------
| 1 | Value1 |
| 2 | Value2 |
| 3 | Value3 |
---------------
Note that values of Orders.SKUField1 and Orders.SKUField2 come from the Products.SKU_NUM values
Actual Query:
SELECT /*+ gather_plan_statistics */ Id, PId, SKUField1, SKUField2, SKU_NUM
FROM Orders
LEFT JOIN (
-- this inner query reduces size of Products from 2 million rows down to 1462 rows
select * from Products where SKU_NUM in (
select SKUField1 from Orders
)
) p1 ON SKUField1 = p1.SKU_NUM
LEFT JOIN (
-- this inner query reduces size of table B from 2 million rows down to 459 rows
select * from Products where SKU_NUM in (
select SKUField2 from Orders
)
) p4 ON SKUField2 = p4.SKU_NUM
OFFSET 5 ROWS FETCH NEXT 10 ROWS ONLY
Execution Plan:
--------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.06 | 8013 | | | |
|* 1 | VIEW | | 1 | 00:00:01 | 10 |00:00:00.06 | 8013 | | | |
|* 2 | WINDOW NOSORT STOPKEY | | 1 | 00:00:01 | 15 |00:00:00.06 | 8013 | 27M| 1904K| |
|* 3 | HASH JOIN RIGHT OUTER | | 1 | 00:00:01 | 15 |00:00:00.06 | 8013 | 1162K| 1162K| 1344K (0)|
| 4 | VIEW | | 1 | 00:00:01 | 1462 |00:00:00.04 | 6795 | | | |
| 5 | NESTED LOOPS | | 1 | 00:00:01 | 1462 |00:00:00.04 | 6795 | | | |
| 6 | NESTED LOOPS | | 1 | 00:00:01 | 1462 |00:00:00.04 | 5333 | | | |
| 7 | SORT UNIQUE | | 1 | 00:00:01 | 1469 |00:00:00.04 | 3010 | 80896 | 80896 |71680 (0)|
| 8 | TABLE ACCESS FULL | Orders | 1 | 00:00:01 | 113K|00:00:00.02 | 3010 | | | |
|* 9 | INDEX UNIQUE SCAN | UIX_Product_SKU_NUM | 1469 | 00:00:01 | 1462 |00:00:00.01 | 2323 | | | |
| 10 | TABLE ACCESS BY INDEX ROWID | Products | 1462 | 00:00:01 | 1462 |00:00:00.01 | 1462 | | | |
|* 11 | HASH JOIN RIGHT OUTER | | 1 | 00:00:01 | 15 |00:00:00.02 | 1218 | 1142K| 1142K| 1335K (0)|
| 12 | VIEW | | 1 | 00:00:01 | 459 |00:00:00.02 | 1213 | | | |
| 13 | NESTED LOOPS | | 1 | 00:00:01 | 459 |00:00:00.02 | 1213 | | | |
| 14 | NESTED LOOPS | | 1 | 00:00:01 | 459 |00:00:00.02 | 754 | | | |
| 15 | SORT UNIQUE | | 1 | 00:00:01 | 462 |00:00:00.02 | 377 | 24576 | 24576 |22528 (0)|
| 16 | INDEX FAST FULL SCAN | Orders_SKUField2_IDX6 | 1 | 00:00:01 | 113K|00:00:00.01 | 377 | | | |
|* 17 | INDEX UNIQUE SCAN | UIX_Product_SKU_NUM | 462 | 00:00:01 | 459 |00:00:00.01 | 377 | | | |
| 18 | TABLE ACCESS BY INDEX ROWID| Products | 459 | 00:00:01 | 459 |00:00:00.01 | 459 | | | |
| 19 | TABLE ACCESS FULL | Orders | 1 | 00:00:01 | 15 |00:00:00.01 | 5 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------
Hence, based on the "A-Rows" column values for row Ids 8 and 16 in the execution plan, it seems like there are full table scans on the Orders table (though row id 16 atleast seems to be using an index). So my question is is it true that there is a full table scan on the orders table even though I am using Offset/Fetch Next

Answer 1

Although your FETCH clause may use a full table scan, Oracle will still only fetch the first X rows from the table.
In the following example, the "TABLE ACCESS FULL" operation does start to read the entire table, but it gets cutoff part of the way through by the "WINDOW NOSORT STOPKEY" operation. Not all full table scans actually scan the full table. You would see similar behavior if your code ended with WHERE ROWNUM <= 50.
CREATE TABLE some_table AS SELECT * FROM all_objects;
EXPLAIN PLAN FOR SELECT * FROM some_table OFFSET 5 ROWS FETCH NEXT 10 ROWS ONLY;
SELECT * FROM TABLE(dbms_xplan.display);
Plan hash value: 2559837639
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 15 | 7410 | 2 (0)| 00:00:01 |
|* 1 | VIEW | | 15 | 7410 | 2 (0)| 00:00:01 |
|* 2 | WINDOW NOSORT STOPKEY| | 15 | 2010 | 2 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | SOME_TABLE | 15 | 2010 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=15 AND
"from$_subquery$_002"."rowlimit_$$_rownumber">5)
2 - filter(ROW_NUMBER() OVER ( ORDER BY NULL )<=15)
The performance implications get more complicated if you also want to order the results. If that is the case, you may want to post the full query and execution plan.
(EDIT: 2022-09-25)
Yes, there is a full table scan on the ORDERS table happening on line 8 of the execution plan. As you mentioned, you can look at the "A-rows" column to tell what's really happening.
But the third full table scan of ORDERS, on line 19, is not a "full" full table scan. The operation "WINDOW NOSORT STOPKEY" stops that full table scan as soon as the 15 necessary rows are read. So the FETCH syntax is helping at least a little.
Applying a FETCH to a query does not mean that every single table will be limited. Although, in your query, it does seem like there ought to be a way to reduce the full table scans. Perhaps an index on SKUField1 would help?

Answer 2

Since Oracle as I know don't provide something like limit or top you can created by yourself like the following:
what is happening here, the inner query gets all the first 10 records and the outer query get those, you can still use any clauses like where or order or any others
SELECT * FROM (
SELECT * FROM Customers WHERE CustomerID <= 10 ORDER BY CustomerID
)
The full article will be found about this topic here at Oracle-Fetch
I am using Online Oracle so you can try it from your end, please let me know if you still have a problem.