Why this lesson matters

Understand reading query plans and reason about it with a concrete relational example. Indexes trade additional storage and write work for faster access paths. Their value depends on predicates, selectivity, ordering and the query plan chosen by the database.

How to reason about it

  • For Reading Query Plans, the outcome to verify is: Run the statement against sample data and inspect the result before moving to the next case.
  • In Reading Query Plans, keep this failure controlled: Adding an index for every column increases write cost and maintenance without guaranteeing the optimizer will use it.
  • Reading Query Plans practice target: Compare EXPLAIN output before and after an index on a selective filter, then note how rows scanned and access method change.

Practical walkthrough

In the Reading Query Plans walkthrough: Run the statement against sample data and inspect the result before moving to the next case.

example.sqlsql
EXPLAIN
SELECT * FROM orders
WHERE customer_id = 101;

Practice it yourself

Practice

Reading Query Plans exercise

Compare EXPLAIN output before and after an index on a selective filter, then note how rows scanned and access method change.

  • Record the expected result before execution
  • Test one valid path and one lesson-specific failure path
  • Explain in two lines which boundary owns the decision

Summary