9.Parsing Engine

The Parsing Engine

A

Parsing Engine (PE) is a virtual processor (vproc). It is made up of the

following software components: Session Control, the Parser, the

Optimizer, and the Dispatcher.

Once a valid session has been established, the PE is the component that

manages the dialogue between the client application and the RDBMS.

Each PE can handle up to 120 sessions, and each session can handle

multiple requests.

Session Control

The major functions performed by Session Control are logon and logoff.

Logon takes a textual request for session authorization, verifies it, and

returns a yes or no answer. Logoff terminates any ongoing activity and

deletes the session’s context.

Parser

The Parser interprets SQL statements, checks them for proper SQL

syntax and evaluates them semantically. The PE also consults the Data

Dictionary to ensure that all objects and columns exist and that the user

has authority to access these objects.

Optimizer

The

Optimizer is responsible for developing the least expensive plan to

return the requested response set. Processing alternatives are evaluated

and the fastest plan is chosen. This plan is converted to executable steps,

to be performed by the AMPs, which are then passed to the dispatcher. In

order to maximize throughput and minimize resource contention, the

optimizer must know about

system configuration, available units of

parallelism

(AMPs and PE's), and data demographics. The Teradata

Optimizer is robust and intelligent. The optimizer enables Teradata to

handle multiple complex, ad hoc queries efficiently. It is

parallel-aware

and

cost-based and uses full look-ahead capability.

Dispatcher

The Dispatcher controls the sequence in which the steps are executed

and passes the steps on to the BYNET. It is composed of executioncontrol

and response-control tasks. Execution control receives the step

definitions from the Parser and transmits them to the appropriate AMP(s)

for processing, receives status reports from the AMPs as they process the

steps, and passes the results on to response control once the AMPs have

completed processing. Response control returns the results to the user.

The Dispatcher sees that all AMPs have finished a step before the next

step is dispatched. Depending on the nature of the SQL request, a step

will be sent to one AMP, or broadcast to all AMPs.