Distributed DBMS - CS712 Power Point Slides Lecture 35

semijoin is better if sj with more than two sj tables

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Unformatted text preview: utes R’ ⋈ S • Ignoring Tmsg semijoin is better if better –Size(πA(S)) + size(R ⋉A S) < size(R) • Join is better if …..• Semijoin is better if…..-. • SJ with more than two SJ tables Will be more complex complex • Semijoin approach can be Semijoin applied to each individual join, consider EMP ⋈ ASG ⋈ PROJ • EMP ⋈ ASG ⋈ PROJ = • EMP’ ⋈ ASG’ ⋈ PROJ where • EMP’ = EMP ⋉ ASG and • ASG’ = ASG ⋉ PROJ rather • EMP” = EMP ⋉ (ASG ⋉ PROJ) • Many SJ expressions Many possible for a relation possible • “Full reducer” a SJ Full expression that reduces R the maximum the • Not exists for cyclic Not queries. queries. • Select eName From Select EMP, ASG, PROJ Where EMP.eNo = ASG....
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This document was uploaded on 02/26/2014.

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