Query Evaluation (cont)
We call these specialised version of RA operations RelOps.
One major task of the query processor:
- given a RA expression to be evaluated
- find a combination of RelOps to do this efficiently
Requires the query translator/optimiser to consider
- information about relations (e.g. sizes, primary keys, ...)
- information about operations (e.g. selection reduces size)
RelOps are realised at execution time
- as a collection of inter-communicating nodes
- communicating either via pipelines or temporary relations
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