- Example Query Tree:
- Fact:
- A query consists of a hierarchy (= tree) of physical (query plan) operators
- Data flow:
- The result of the operators at the lower level in the tree must be passed (up) to some operator at a higher level in the tree
- There are 2 techniques
used to
pass the
result of
one operator to
another operator:
- Materialization:
- The output of the
operator is
written
(as a temporal relation) to
disk
- The receiving operator will read the tuples from disk
Graphically: σsalary > 50000 (employee) ⋈dno = dnumber σdname = 'Research' (department)
Advantage/disadvantages:
- Simple to
implement
- High I/O costs....
- Low memory size
requirement
(because we do not use
extra memory buffers)
- Very attractive for small memory computer systems
- The output of the
operator is
written
(as a temporal relation) to
disk
- Pipelining:
- The output of the
operator is
stored
in one or more
memory buffer(s)
- The receiving operator will read the tuples from the memory buffer
Graphically: σsalary > 50000 (employee) ⋈dno = dnumber σdname = 'Research' (department)
Advantage/disadvantages:
- Harder to
implement
- The commonly used technique
to implementing pipelining is:
- Iterator objects
- The commonly used technique
to implementing pipelining is:
- Lower I/O costs....
- High memory
requirement
(because we need extra memory buffers)
- Attractive for large memory computer systems
- The output of the
operator is
stored
in one or more
memory buffer(s)
- Materialization: