Databases Reference
In-Depth Information
C
π
ename
Q
σ
salary > X
Δ
S
Q
C
π
ename
Q
h
Map
cond
(salary > X
)
salary > X
ω
emp
S
S
Δ
Δ
Q
γ
AVG
PH
(salary
h
)
→
X
h
emp
S
γ
AVG
PH
(salary
h
)
→
X
h
σ
did
f
=E(1)
σ
did
f
=E(1)
emp
S
S
emp
Fig. 3.
Original query tree
Fig. 4.
Replacing encrypted rela-
tions
Fig. 5.
Doing selection at server
Fig. 6.
Multiple interactions between
client and server
server. That is, select the tuples corresponding to the employees that work in
department
did
= 1. The results are shipped to the client, which decrypts the
tuples and computes average salary. The average salary is encrypted by the
client and shipped back to the server, which then computes the join at the
server. Finally, the results are decrypted at the client.
Supporting Aggregation Operators in Queries
:
The various query translation
techniques discussed above are designed explicitly for relational operators that
