Sunday, January 16, 2011

What do High confidence, Low confidence and No confidence mean in EXPLAIN plan?

What do High confidence, Low confidence and No confidence mean in EXPLAIN plan?
Answer

Explain gives the execution strategy means what are the different steps that the query will go through.
HIGH CONFIDENCE:  Statistics are collected.
LOW CONFIDENCE:   Statistics are not collected.  But the where condition is having the condition on indexed column. Then estimations can be based on sampling.
NO CONFIDENCE: Statistics are not collected and the condition is on non indexed column.

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