Query Execution:
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Unit of cost: Number of pages read/written

Cost of queries:  All

Main operators/Multiple implementations:
     select/project/join/group by/distinct/union/intersect/except

Each query has some memory allocated, each operator has its own memory
allocation:  Assume M blocks for a given query

Dependence on memory available for operations and how the relations
are stored, the cost of a specific implementation of an operation will
differ

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Size of a relation R, PAGES(R)  and memory allocated M determine the cost

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Iterator interface:

open/close/get_next



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Operator implementations

1. Using indexes

2. Single pass/multi pass  (Block nested join)

3. Temporary outputs  (External sort)