Database Reference
In-Depth Information
Bushy Trees
A
bushy tree
, as shown in Figure
14-5
, is a join tree that might have a join with two inputs that aren't tables. In other
words, the structure of the tree is completely free. The query optimizer chooses this type of join tree only if it has no
other possibility. This usually happens when unmergeable views or subqueries are present.
Figure 14-5.
The structure of a bushy tree is completely free
The following execution plan illustrates the join tree depicted in Figure
14-5
. Notice that the children of join
operation 1 are result sets of two other join operations:
-------------------------------------
| Id | Operation | Name |
-------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN | |
| 2 | VIEW | |
| 3 | HASH JOIN | |
| 4 | TABLE ACCESS FULL| T1 |
| 5 | TABLE ACCESS FULL| T2 |
| 6 | VIEW | |
| 7 | HASH JOIN | |
| 8 | TABLE ACCESS FULL| T3 |
| 9 | TABLE ACCESS FULL| T4 |
-------------------------------------
