Databases Reference
In-Depth Information
(Contact join Individual) join Customer - > Contact join(Individual join Customer)
Listing 5-20.
or
(1 join 2) join 4 - > 1 join(2 join 4)
Listing 5-21.
... we obtain two new operators that are not yet present in the memo structure. The first
one is
Join 2 & 4
. Since we do not have an equivalent group for this new operator, we
place it in a new group, Group 5 in this case. The second operator joins Group 1 and the
new operator we just added to the memo structure. This new operator would be
Join 1 &
5
, and since
Join 1 & 5
is equivalent to
Join 3 & 4,
we place them in the same group, which
is also the root group. These two changes are shown in Table 5-8.
Group 6
Join 3 & 4
Join 1 & 5
Group 5
Join 2 & 4
Group 4
Scan Customer
Group 3
Join 1 & 2
Group 2
Scan Individual
Group 1
Scan Contact
Table 5-8:
Memo structure after associativity rule has been applied.
