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Published bySimon Meadowcroft
Modified over 2 years ago
Independent consultant Available for consulting In-house workshops Cost-Based Optimizer Performance By Design Performance Troubleshooting Oracle ACE Director Member of OakTable Network
Three main questions you should ask when looking for an efficient execution plan: How much data? How many rows / volume? How scattered / clustered is the data? Caching? => Know your data!
Why are these questions so important? Two main strategies: One Big Job => How much data, volume? Few/many Small Jobs => How many times / rows? => Effort per iteration? Clustering / Caching
Optimizers cost estimate is based on: How much data? How many rows / volume? How scattered / clustered? (partially) (Caching?) Not at all
Clustering Factor Statistics / Histograms Datatype issues
1,000 rows => visit 1,000 table blocks: 1,000 * 5ms = 5 s
1,000 rows => visit 10 table blocks: 10 * 5ms = 50 ms
Re-visiting the same recent table blocks
SELECT SKEWED_NUMBER FROM T ORDER BY SKEWED_NUMBER 1,000 2,000 3,000 4,000 5,000 … 1 70, , , , ,000 10,000,000 rows 100 popular values with 70,000 occurrences 250 buckets each covering 40,000 rows (compute) 250 buckets each covering approx. 22/23 rows (estimate) 40,000 80, , , , , , , , ,000 1,000 2,000 3,000 4,000 5,000 6,000 Rows Endpoint
1, ,000 7,000,000 10,000,000
1, ,000 7,000,000 10,000,000
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