3 times FTS of a simple "connect by" hierarchy query

Question

Please have a look at the query and its explain plan. In the query emp table has been used once.
But, FTS is happening 3 times. Why is so? What is the significance of "CONNECT BY PUMP" clause?
1. SELECT ename,LEVEL lv
FROM emp
CONNECT BY mgr = PRIOR empno
START WITH mgr IS NULL
order siblings by ename;
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
| 0 | SELECT STATEMENT | | 14 | 196 | 4 (25)|
| 1 | CONNECT BY WITH FILTERING| | | | |
| 2 | FILTER | | | | |
| 3 | TABLE ACCESS FULL | EMP | 14 | 196 | 3 (0)|
| 4 | HASH JOIN | | | | |
| 5 | CONNECT BY PUMP | | | | |
| 6 | TABLE ACCESS FULL | EMP | 14 | 196 | 3 (0)|
| 7 | TABLE ACCESS FULL | EMP | 14 | 196 | 3 (0)|
Thanks in advance,
Amit

Answer 1

When Oracle determines the best way to access a table, it looks at various information and weighs them up against each other. If all the table is so small that the data can be read in a single physical block read then it may as well just load it all and will treat that as a FTS. In such a case, for it to go off and read an index it would likely take a block read to read the index and then another block read to obtain the data, which is more accesses than a FTS.
Full Tables Scans are not always a bad thing, despite what some teachers may say.