A particular sort order is said to be interesting-sort order if it could be useful for a later operation.

The join orders where the right operand of each join is in one of the initial relations are called as left deep join orders.

Cost based optimizers may be expensive as exploring space of all possible plans might prove to be costly.

The process of replacing a nested query with a query with a join is known as decorrelation. It is more complicated when the nested sub query uses aggregate functions inside it.

Cost based optimizers may be expensive as exploring space of all possible plans might prove to be costly. So, optimizers use heuristics to reduce the cost of optimization.

Caching and reuse of query plans is called as plan caching. It is used because the optimal plan for the new constants may differ from the optimal plan for the initial values.

A cost based optimizer explores the space of all query evaluation plans that are equivalent to a given query, and chooses the one with the least estimated cost.

The technique used to evaluate a query with a nested sub query is called as correlated evaluation. It is not very efficient as a large number of I/O operations may occur.

For promoting space efficiency, making multiple copies of the same sub-expressions must be avoided.

The rule that allows transformation of a logical operation to a physical operation is called as a physical equivalence rule. It is a new class of equivalence rules.