written 7.4 years ago by | • modified 2.5 years ago |

**Subject:** Distributed Database

**Topic:** Distributed query processing and optimization

**Difficulty:** High

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What is query optimisation? List distributed Query optimisation algorithms and explain any one from that.

written 7.4 years ago by | • modified 2.5 years ago |

**Subject:** Distributed Database

**Topic:** Distributed query processing and optimization

**Difficulty:** High

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written 7.4 years ago by |

**Query Optimization**

Query optimization is the process of selecting the most efficient quey-evaluation plan from among the many strategies usually possible for a given query. It does not expect users to write their queries so that they can be processed efficiently. Rather, it expects the system to construct a query evaluation plan that minimizes the cost of query evaluation.

**Distributed query optimization algorithm:**

- Semi-join Algorithm
- INGRES Algorithm
- System R Algorithm
- System R* Algorithm
- Hill Climbing Algorithm
- SDD-1 Algorithm

**Hill Climbing Algorithm**

- It refinements of an initial feasible solution are recursively computed until no more cost improvements can be made.
- In hill climbing algorithm, Semijoins, data replication, and fragmentation are not used.
- It devised for wide area point-to-point networks.
- It is the first distributed query processing algorithm.
The hill-climbing algorithm proceeds as follows:

i. Select initial feasible execution strategy ES0

• i.e., a global execution schedule that includes all intersite communication

• Determine the candidate result sites, where a relation referenced in the query exist

• Compute the cost of transferring all the other referenced relations to each candidate site

• ES0 = candidate site with minimum cost

ii. Split ES0 into two strategies: ES1 followed by ES2

• ES1: send one of the relations involved in the join to the other relation’s site –

• ES2: send the join result to the final result site

iii. Replace ES0 with the split schedule which gives

iv. Recursively apply steps 2 and 3 on ES1 and ES2 until no more benefit can be gained

v. Check for redundant transmissions in the final plan and eliminate them

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