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Estimating the cost of a plan

Table statistics

Estimating selectivity


select *
from Employee
where age between 65 and 70

Use the index or scan the table?

There are 710 employees.
between 65 and 70 is covered by the last two buckets.
- Assuming uniform distribution within the 63-67 bucket, 65-67 is 3/5 of the bucket, so 19.2 Employees.
- Similarly, 68-70 is 3/5 of the 68-72 bucket: 4.2
- Total: 23.4
Selectivity is 23.4 / 710 = 3.3%.

OK to use an index up to 10% selectivity, so: Use the index

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