How do you read and interpret a SQL Query Execution Plan?
An Execution Plan is the sequence of operations chosen by the database query optimizer to execute a SQL statement. You inspect it using `EXPLAIN` or `EXPLAIN ANALYZE`.
Key components to look for: 1. Scan Type: 'Index Scan' or 'Index Seek' is fast; 'Seq Scan' or 'Table Scan' checks every row. 2. Cost / Time: Optimizer estimate vs actual execution time. 3. Rows: Estimated rows vs actual rows returned (a large discrepancy indicates stale table statistics). 4. Joins: Nested Loop (good for small sets with index), Hash Join (good for large unindexed sets), or Merge Join (good for presorted sets).
-- Inspect actual runtime execution metrics in PostgreSQL
EXPLAIN ANALYZE
SELECT u.name, o.total_amount
FROM Users u
JOIN Orders o ON u.id = o.user_id
WHERE o.created_at >= '2026-01-01'
AND u.status = 'VIP';Reading the tree output from innermost to outermost reveals bottlenecks: for example, if the engine performed a full table scan on `Orders` instead of using an index on `created_at`.
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