![Semijoin Algorithms (cont.)
• Consider the join of two relations
• R[A] (located at site 1)
• S[A] (located at site 2)
• Alternatives
• 1. Do the join R ⋈A S
• 2. Perform one of the semijoin equivalents
( ) ( )
( ) ( )
A A A A A
A A A
R S R S S R S R
R S S R
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Join ordering in fragment queries
- 1. Join Ordering in Fragment Queries By Shehab Uddin and Ifzal Hussain
- 2. Join Ordering in Fragment Queries • Join ordering is important in centralized DB, and is more important in distributed DB.
- 3. Join Ordering in Fragment Queries (cont.) • R site j: “relation R is transferred to site j” • 1. EMP site 2; site 2 computes EMP’ • EMP’->site 3; site 3 computes the result. • 2.ASG->site 1: site 1 computes EMP’, EMP’->site 3; site 3 computes the result • 3. ASG->site 3; computeASG’;ASG’->site 1 • 4. PROJ->site 2; compute PROJ’; PROJ’->site 1 • 5. EMP->site 2; PROJ->site 2; site 2 compute the join.
- 4. Join Ordering in Fragment Queries (cont.) • Join ordering • Distributed INGRES • System R* • Semijoin ordering • SDD-1
- 5. Join Ordering • Consider two relations only • R ⋈ S • Transfer the smaller size • Multiple relations more difficult because too many alternatives • Compute the cost of all alternatives and select the best one • Necessary to compute the size of intermediate relations which is difficult. • Use heuristics
- 6. Join Ordering - Example • Consider: PROJ ⋈PNO ASG ⋈ENO EMP
- 7. Join Ordering – Example (cont.) • Execution alternatives: • 1. EMP Site 2 • Site 2 computes EMP’=EMP⋈ASG • EMP’ Site 3 • Site 3 computes EMP’⋈PROJ • 2.ASG Site 1 • Site 1 computes EMP’=EMP⋈ASG • EMP’ Site 3 • Site 3 computes EMP’⋈PROJ
- 8. Join Ordering – Example (cont.) 3. ASG Site 3 Site 3 computes ASG’=ASG⋈PROJ ASG’ Site 1 Site 1 computes ASG’⋈EMP 4. PROJ Site 2 Site 2 computes PROJ’=PROJ⋈ASG PROJ’ Site 1 Site 1 computes PROJ’ ⋈ EMP
- 9. cont,d 5. EMP Site 2 PROJ Site 2 Site 2 computes EMP⋈ PROJ⋈ASG
- 10. Semijoin Algorithms • Shortcoming of the joining method • Transfer the entire relation which may contain some useless tuples • Semi-join reduces the size of operand relation to be transferred • Semi-join is beneficial if the cost to produce and send to the other site is less than sending the whole relation.
- 11. Semijoin Algorithms (cont.) • Consider the join of two relations • R[A] (located at site 1) • S[A] (located at site 2) • Alternatives • 1. Do the join R ⋈A S • 2. Perform one of the semijoin equivalents ( ) ( ) ( ) ( ) A A A A A A A A R S R S S R S R R S S R
- 12. Cnt,d • Perform the join • Send R to site 2 • Site 2 computes R ⋈A S • Consider semijoin • S’ = A(S) • S’ Site 1 • Site 1 computes • R’ Site 2 • Site 2 computes • Semijoin is better if ( ) A A R S S ' ' A R R S ' A R S ( ( ( )) ( )) ( ) A A size S size R S size R