Figure 3.5: Memo Structure for Query 3.2: Join of 3 Relations

           Figure 3.5 shows the memo structure from figure 3.3, including the physical expressions. In this simple case, most logical expressions have a single corresponding physical expression. In group 4, each logical expression with the logical operator EQJOIN has a corresponding physical expression with the physical operator HASH_JOIN. In general, this simple correspondence will not hold. For example, each logical expression could result in multiple physical expressions, each corresponding to a different algorithm for implementing the logical operation. Groups 0, 1 and 2 illustrate this.

3. Fundamental Concepts:	<Models> <Log> <Query Trees> <Groups> <Memo> <Phys> <Item> <Rules> <Opt Stages> <Find Best> <Complexity>   3.1: EQJOIN     3.2a: Query Tree     3.2b: Equiv.     3.3a: Init Memo     3.3b: Memo     3.3c: Key     3.4: Complexity     3.5: Phys Memo     3.6: Plan     3.7: Pred.     3.8: Op Types     3.9: Rule

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