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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/1 Outline Introduction Background Distributed Database Design Database Integration Semantic Data Control Distributed Query Processing ➡ Overview ➡ Query decomposition and localization ➡ Distributed query optimization Multidatabase query processing Distributed Transaction Management Data Replication Parallel Database Systems Distributed Object DBMS Peer-to-Peer Data Management Web Data Management Current Issues

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/2 Query Processing in a DDBMS high level user query query processor Low-level data manipulation commands for D-DBMS

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/3 Distributed Query Processing Methodology Calculus Query on Distributed Relations CONTROL SITE LOCAL SITES Query Decomposition Query Decomposition Data Localization Data Localization Algebraic Query on Distributed Relations Global Optimization Global Optimization Fragment Query Local Optimization Local Optimization Optimized Fragment Query with Communication Operations Optimized Local Queries GLOBAL SCHEMA GLOBAL SCHEMA FRAGMENT SCHEMA FRAGMENT SCHEMA STATS ON FRAGMENTS STATS ON FRAGMENTS LOCAL SCHEMAS LOCAL SCHEMAS

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/4 Step 1 – Query Decomposition Input : Calculus query on global relations Normalization ➡ manipulate query quantifiers and qualification Analysis ➡ detect and reject “incorrect” queries ➡ possible for only a subset of relational calculus Simplification ➡ eliminate redundant predicates Restructuring ➡ calculus query algebraic query ➡ more than one translation is possible ➡ use transformation rules

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/5 Normalization Lexical and syntactic analysis ➡ check validity (similar to compilers) ➡ check for attributes and relations ➡ type checking on the qualification Put into normal form ➡ Conjunctive normal form ( p 11 p 12 … p 1 n ) … ( p m 1 p m 2 … p mn ) ➡ Disjunctive normal form ( p 11 p 12 … p 1 n ) … ( p m 1 p m 2 … p mn ) ➡ OR's mapped into union ➡ AND's mapped into join or selection

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/6 Analysis Refute incorrect queries Type incorrect ➡ If any of its attribute or relation names are not defined in the global schema ➡ If operations are applied to attributes of the wrong type Semantically incorrect ➡ Components do not contribute in any way to the generation of the result ➡ Only a subset of relational calculus queries can be tested for correctness ✦ Those that do not contain disjunction and negation ➡ To detect ✦ connection graph (query graph) ✦ join graph

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/7 Analysis – Example SELECTENAME,RESP FROMEMP, ASG, PROJ WHEREEMP.ENO = ASG.ENO AND ASG.PNO = PROJ.PNO ANDPNAME = "CAD/CAM" ANDDUR ≥ 36 ANDTITLE = "Programmer" Query graph Join graph DUR≥36 PNAME=“CAD/CAM” ENAME EMP.ENO=ASG.ENO ASG.PNO=PROJ.PNO RESULT TITLE = “Programmer” RESP ASG.PNO=PROJ.PNO EMP.ENO=ASG.ENO ASGPROJEMP PROJASG

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/8 Analysis If the query graph is not connected, the query may be wrong or use Cartesian product SELECTENAME,RESP FROMEMP, ASG, PROJ WHEREEMP.ENO = ASG.ENO ANDPNAME = "CAD/CAM" ANDDUR > 36 ANDTITLE = "Programmer" PNAME=“CAD/CAM” ENAME RESULT RESP ASGPROJEMP

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/9 Simplification Why simplify? ➡ Remember the example How? Use transformation rules ➡ Elimination of redundancy ✦ idempotency rules p 1 ¬( p 1 ) false p 1 ( p 1 p 2 ) p 1 p 1 false p 1 … ➡ Application of transitivity ➡ Use of integrity rules

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/10 Simplification – Example SELECTTITLE FROMEMP WHEREEMP.ENAME = "J. Doe" OR(NOT(EMP.TITLE = "Programmer") AND(EMP.TITLE = "Programmer" OREMP.TITLE = "Elect. Eng.") ANDNOT(EMP.TITLE = "Elect. Eng.")) SELECTTITLE FROMEMP WHEREEMP.ENAME = "J. Doe"

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/11 Restructuring Convert relational calculus to relational algebra Make use of query trees Example Find the names of employees other than J. Doe who worked on the CAD/CAM project for either 1 or 2 years. SELECTENAME FROMEMP, ASG, PROJ WHEREEMP.ENO = ASG.ENO ANDASG.PNO = PROJ.PNO ANDENAME≠ "J. Doe" ANDPNAME = "CAD/CAM" AND(DUR = 12 OR DUR = 24) ENAME σ DUR=12 OR DUR=24 σ PNAME=“CAD/CAM” σ ENAME≠“J. DOE” PROJASGEMP Project Select Join ⋈ PNO ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/12 Restructuring –Transformation Rules Commutativity of binary operations ➡ R × S S × R ➡ R ⋈ S S ⋈ R ➡ R S S R Associativity of binary operations ➡ ( R × S ) × T R × ( S × T ) ➡ ( R ⋈ S ) ⋈ T R ⋈ ( S ⋈ T ) Idempotence of unary operations ➡ A ’ ( A ’ ( R )) A ’ ( R ) ➡ p 1 ( A 1 ) ( p 2 ( A 2 ) ( R )) p 1 ( A 1 ) p 2 ( A 2 ) ( R ) where R [ A ] and A' A, A" A and A' A" Commuting selection with projection

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/13 Restructuring – Transformation Rules Commuting selection with binary operations ➡ p ( A ) ( R × S ) ( p ( A ) ( R )) × S ➡ p ( A i ) ( R ⋈ ( A j,B k ) S ) ( p ( A i ) ( R )) ⋈ ( A j,B k ) S ➡ p ( A i ) ( R T ) p ( A i ) ( R ) p ( A i ) ( T ) where A i belongs to R and T Commuting projection with binary operations ➡ C ( R × S ) A ’ ( R ) × B ’ ( S ) ➡ C ( R ⋈ ( A j,B k ) S ) A ’ ( R ) ⋈ ( A j,B k ) B ’ ( S ) ➡ C ( R S ) C ( R ) C ( S ) where R [ A ] and S [ B ]; C = A ' B ' where A' A, B' B

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/14 Example Recall the previous example: Find the names of employees other than J. Doe who worked on the CAD/CAM project for either one or two years. SELECT ENAME FROMPROJ, ASG, EMP WHEREASG.ENO=EMP.ENO ANDASG.PNO=PROJ.PNO ANDENAME ≠ "J. Doe" ANDPROJ.PNAME="CAD/CAM" AND(DUR=12 OR DUR=24) ENAME DUR=12 DUR=24 PNAME=“CAD/CAM” ENAME≠“J. DOE” PROJASGEMP Project Select Join ⋈ PNO ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/15 Equivalent Query ENAME PNAME=“CAD/CAM” (DUR=12 DUR=24) ENAME≠“J. Doe” × PROJ ASG EMP ⋈ PNO,ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/16 EMP ENAME ENAME ≠ "J. Doe" ASGPROJ PNO,ENAME PNAME = "CAD/CAM" PNO DUR =12 DUR=24 PNO,ENO PNO,ENAME Restructuring ⋈ PNO ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/17 Step 2 – Data Localization Input: Algebraic query on distributed relations Determine which fragments are involved Localization program ➡ substitute for each global query its localized query ✦ A localized query is a relational algebra query whose operands are the fragments of relations instead of the relations themselves ✦ We call these operands that are fragments of relations “ localization programs ” ✓ Union for horizontal fragmentation; Join for vertical fragmentation ✦ Replication is not taken into account in this chapter ➡ Optimize ✦ For each type of fragmentation, use reduction techniques to generate simpler queries ✦ To do so, use appropriate heuristics

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/18 Example Assume ➡ EMP is fragmented into EMP 1, EMP 2, EMP 3 as follows: ✦ EMP 1 = ENO≤“E3” (EMP) ✦ EMP 2 = “E3”

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/19 Reduction for PHF Reduction with selection ➡ Relation R and F R ={ R 1, R 2, …, R w } where R j = p j ( R ) p i ( R j )= if x in R : ¬( p i ( x ) p j ( x )) ➡ Example SELECT* FROMEMP WHEREENO="E5" ENO=“E5” EMP 1 EMP 2 EMP 3 EMP 2 ENO=“E5”

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/20 Reduction for PHF Reduction with join ➡ Possible if fragmentation is done on join attribute, i.e., the selection attribute used for the fragmentation is the same as the join attribute ➡ Algorithm ✦ Distribute joins over unions ( R 1 R 2 ) ⋈ S ( R 1 ⋈ S ) ( R 2 ⋈ S ) ✦ Eliminate useless joins as follows: Given R i = p i ( R ) and R j = p j ( R ) R i ⋈ R j = if x in R i, y in R j : ¬( p i ( x ) p j ( y )) That is, qualifications of the joined fragments are in contradiction

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/21 Reduction for PHF Assume EMP is fragmented as before and ➡ ASG 1 : ENO ≤ "E3" (ASG) ➡ ASG 2 : ENO > "E3" (ASG) Consider the query SELECT* FROMEMP,ASG WHEREEMP.ENO=ASG.ENO Distribute join over unions Apply the reduction rule EMP 1 EMP 2 EMP 3 ASG 1 ASG 2 ⋈ ENO EMP 1 ASG 1 EMP 2 ASG 2 EMP 3 ASG 2 ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/22 Provides Parallellism EMP 3 ASG 1 EMP 2 ASG 2 EMP 1 ASG 1 EMP 3 ASG 2 ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/23 Eliminates Unnecessary Work EMP 2 ASG 2 EMP 1 ASG 1 EMP 3 ASG 2 ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/24 Reduction for VF Find useless (not empty) intermediate relations Relation R defined over attributes A = { A 1,..., A n } vertically fragmented as R i = A ' ( R ) where A ' A : D,K ( R i ) is useless if the set of projection attributes D is not in A ' Example: EMP 1 = ENO,ENAME (EMP); EMP 2 = ENO,TITLE (EMP) SELECTENAME FROMEMP EMP 1 EMP 2 ENAME ⋈ ENO ENAME

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/25 Reduction for DHF Rule : ➡ Distribute joins over unions ➡ Apply the join reduction for horizontal fragmentation (using the qualification of the primary fragments!) Example ASG 1 : ASG ⋉ ENO EMP 1 ASG 2 : ASG ⋉ ENO EMP 2 EMP 1 : TITLE=“Programmer” (EMP) EMP 2 : TITLE≠“Programmer” (EMP) Query SELECT * FROMEMP, ASG WHEREASG.ENO = EMP.ENO ANDEMP.TITLE = "Mech. Eng."

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/26 Generic query Selections first Reduction for DHF ASG 1 TITLE=“Mech. Eng.” ASG 2 EMP 1 EMP 2 ASG 1 ASG 2 EMP 2 TITLE=“Mech. Eng.” ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/27 Joins over unions Reduction for DHF Elimination of the empty intermediate relations (left sub-tree) ASG 1 EMP 2 TITLE=“Mech. Eng.” ASG 2 TITLE=“Mech. Eng.” ASG 2 EMP 2 TITLE=“Mech. Eng.” ⋈ ENO

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/28 Reduction for Hybrid Fragmentation Combine the rules already specified: ➡ Remove empty relations generated by contradicting selections on horizontal fragments; ➡ Remove useless relations generated by projections on vertical fragments; ➡ Distribute joins over unions in order to isolate and remove useless joins.

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Distributed DBMS © M. T. Özsu & P. Valduriez Ch.7/29 Reduction for HF Example Consider the following hybrid fragmentation: EMP 1 = ENO≤"E4" ( ENO,ENAME (EMP)) EMP 2 = ENO>"E4" ( ENO,ENAME (EMP)) EMP 3 = ENO,TITLE (EMP) and the query SELECTENAME FROMEMP WHEREENO="E5" EMP 1 EMP 2 EMP 3 ENO=“E5” ENAME EMP 2 ENO=“E5” ENAME ⋈ ENO

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