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Database , 6 Query Introduction
- 1. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/1 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
- 2. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/2 Query Processing in a DDBMS high level user query query processor Low-level data manipulation commands for D-DBMS
- 3. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/3 Query Processing Components • Query language that is used ➡ SQL: “intergalactic dataspeak” • Query execution methodology ➡ The steps that one goes through in executing high-level (declarative) user queries. • Query optimization ➡ How do we determine the “best” execution plan? • We assume a homogeneous D-DBMS
- 4. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/4 SELECT ENAME FROM EMP,ASG WHERE EMP.ENO = ASG.ENO AND RESP = "Manager" Strategy 1 ENAME( RESP=“Manager” EMP.ENO=ASG.ENO(EMP×ASG)) Strategy 2 ENAME(EMP ⋈ENO ( RESP=“Manager” (ASG)) Strategy 2 avoids Cartesian product, so may be “better” Selecting Alternatives
- 5. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/5 What is the Problem? Site 1 Site 2 Site 3 Site 4 Site 5 EMP1= ENO≤“E3”(EMP) EMP2= ENO>“E3”(EMP)ASG2= ENO>“E3”(ASG)ASG1= ENO≤“E3”(ASG) Result Site 5 Site 1 Site 2 Site 3 Site 4 ASG1 EMP1 EMP2ASG2Site 4Site 3 Site 1 Site 2 Site 5 EMP’ 1=EMP1 ⋈ENO ASG’ 1 ' 2 EMPEMPresult ' 1 1Manager""RESP1 ASGσASG ' 2Manager""RESP2 ASGσASG ' ' 1 ASG ' 2 ASG ' 1 EMP ' 2 EMP result= (EMP1 × EMP2)⋈ENOσRESP=“Manager”(ASG1× ASG2) EMP’ 2=EMP2 ⋈ENO ASG’ 2
- 6. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/6 Cost of Alternatives • Assume ➡ size(EMP) = 400, size(ASG) = 1000 ➡ tuple access cost = 1 unit; tuple transfer cost = 10 units • Strategy 1 ➡ produce ASG': (10+10) tuple access cost 20 ➡ transfer ASG' to the sites of EMP: (10+10) tuple transfer cost 200 ➡ produce EMP': (10+10) tuple access cost 2 40 ➡ transfer EMP' to result site: (10+10) tuple transfer cost 200 Total Cost 460 • Strategy 2 ➡ transfer EMP to site 5: 400 tuple transfer cost 4,000 ➡ transfer ASG to site 5: 1000 tuple transfer cost 10,000 ➡ produce ASG': 1000 tuple access cost 1,000 ➡ join EMP and ASG': 400 20 tuple access cost 8,000 Total Cost 23,000
- 7. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/7 Query Optimization Objectives • Minimize a cost function I/O cost + CPU cost + communication cost These might have different weights in different distributed environments • Wide area networks ➡ communication cost may dominate or vary much ✦ bandwidth ✦ speed ✦ high protocol overhead • Local area networks ➡ communication cost not that dominant ➡ total cost function should be considered • Can also maximize throughput
- 8. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/8 Complexity of Relational Operations • Assume ➡ relations of cardinality n ➡ sequential scan Operation Complexity Select Project (without duplicate elimination) O(n) Project (with duplicate elimination) Group O(n log n) Join Semi-join Division Set Operators O(n log n) Cartesian Product O(n2)
- 9. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/9 Query Optimization Issues – Types Of Optimizers • Exhaustive search ➡ Cost-based ➡ Optimal ➡ Combinatorial complexity in the number of relations • Heuristics ➡ Not optimal ➡ Regroup common sub-expressions ➡ Perform selection, projection first ➡ Replace a join by a series of semijoins ➡ Reorder operations to reduce intermediate relation size ➡ Optimize individual operations
- 10. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/10 Query Optimization Issues – Optimization Granularity • Single query at a time ➡ Cannot use common intermediate results • Multiple queries at a time ➡ Efficient if many similar queries ➡ Decision space is much larger
- 11. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/11 Query Optimization Issues – Optimization Timing • Static ➡ Compilation optimize prior to the execution ➡ Difficult to estimate the size of the intermediate results error propagation ➡ Can amortize over many executions ➡ R* • Dynamic ➡ Run time optimization ➡ Exact information on the intermediate relation sizes ➡ Have to reoptimize for multiple executions ➡ Distributed INGRES • Hybrid ➡ Compile using a static algorithm ➡ If the error in estimate sizes > threshold, reoptimize at run time ➡ Mermaid
- 12. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/12 Query Optimization Issues – Statistics • Relation ➡ Cardinality ➡ Size of a tuple ➡ Fraction of tuples participating in a join with another relation • Attribute ➡ Cardinality of domain ➡ Actual number of distinct values • Common assumptions ➡ Independence between different attribute values ➡ Uniform distribution of attribute values within their domain
- 13. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/13 Query Optimization Issues – Decision Sites • Centralized ➡ Single site determines the “best” schedule ➡ Simple ➡ Need knowledge about the entire distributed database • Distributed ➡ Cooperation among sites to determine the schedule ➡ Need only local information ➡ Cost of cooperation • Hybrid ➡ One site determines the global schedule ➡ Each site optimizes the local subqueries
- 14. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/14 Query Optimization Issues – Network Topology • Wide area networks (WAN) – point-to-point ➡ Characteristics ✦ Low bandwidth ✦ Low speed ✦ High protocol overhead ➡ Communication cost will dominate; ignore all other cost factors ➡ Global schedule to minimize communication cost ➡ Local schedules according to centralized query optimization • Local area networks (LAN) ➡ Communication cost not that dominant ➡ Total cost function should be considered ➡ Broadcasting can be exploited (joins) ➡ Special algorithms exist for star networks
- 15. Distributed DBMS © M. T. Özsu & P. Valduriez Ch.6/15 Distributed Query Processing Methodology Calculus Query on Distributed Relations CONTROL SITE LOCAL SITES Query Decomposition Data Localization Algebraic Query on Distributed Relations Global Optimization Fragment Query Local Optimization Optimized Fragment Query with Communication Operations Optimized Local Queries GLOBAL SCHEMA FRAGMENT SCHEMA STATS ON FRAGMENTS LOCAL SCHEMAS