Databases · Senior · Question 55 of 75

What are extended statistics (`CREATE STATISTICS`) for?

1Storing query history
✓Correct answer: Teaching the planner about correlations between columns (e.g. city and zip code), so combined conditions are estimated correctly
3Collecting usage statistics for billing

Explanation

By default the planner assumes columns are independent and multiplies selectivities, which underestimates rows for correlated columns.

Further reading

PostgreSQL: Statistics used by the planner ↗

Test yourself

Take the Databases · Senior quiz: questions and options are shuffled, and you get a review of your mistakes at the end.