## For each plan considered, must estimate cost:

- Must estimate cost of each operation in plan tree.
- Depends on input cardinalities.

- Must estimate size of result for each operation in tree!
- Use information about the input relations.
- For selections and joins, assume independence of predicates.

- Must estimate cost of each operation in plan tree.

## We’ll discuss the System R cost estimation approach.

- Very inexact, but works ok in practice.
- More sophisticated techniques known now.

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