1. Cost-Based Plan Selection Chapter 16 Section 5 ID: 213

2. Outline  Cost-base plan selection involves: Cost estimation Statistics computation Query plan optimization using computed costs: Heuristics approach Enumerating and selecting methods

3. Cost Estimation  Book keeping of counts to estimate costs: T(R), V(R,a), B(R), histogram  Types of Histogram: Equal-width: divide value range by fixed width w and keep counts of each width Equal-height: similar as width but keep aggregated counts Most-frequent-values: values and number of occurences

4. Computation of Statistics  Statistics are fault-tolerant, good for use in estimates; e.g. small error does not significantly change out come.  Inspect the distribution of values across records in relation  Collection on entire relation is expensive; however, can be computed using smaller sample size

5. Reducing Cost by Heuristics  Applies for logical query plan  Estimate cost before and after a transformation  Only choose/apply transformation when cost estimations show beneficial  Example: Deferring duplicate elimination is better

6. Reducing Cost: Example Plan a: 100+50+1000 = 1150 Plan b: 100+1000 = 1100

7. Enumerating Physical Plans  Baseline approach (exhaustive): consider all combinations, pick the smallest cost plan  Other approaches categorized into: Top-down: compute cost from root, take the best Bottom-up: compute cost for all combinations for a sub-expression, select the best, move up until root evaluated

8. List of Other Approaches  Heuristic selection  Branch-and-bound plan enumeration: keep record of best cost, skip to next plan if current plan exceed best known cost  Hill climbing  Dynamic programming (PP): keep least cost of each sub-expression; work bottom-up  Selinger-style optimization: improved version of DP; keep others beneficial plans besides least cost plans

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