Q29: How do you analyze an SQL query execution plan using EXPLAIN?
The EXPLAIN (or EXPLAIN ANALYZE) command reveals the execution steps chosen by the database query optimizer.
Key Metrics to Analyze
- Scan Types:
Seq Scan/Table Scan(slow) vsIndex Seek/Index Scan(fast). - Join Algorithms:
Nested Loop(good for small sets),Hash Join(good for large unsorted sets),Merge Join(efficient for pre-sorted sets). - Cost Estimates: Startup and total cost measures calculated by database statistics engines.
- Actual Time / Rows: Real execution timings provided by
EXPLAIN ANALYZE.
Key Takeaways
EXPLAIN ANALYZEexecutes the target query and reports real runtime performance statistics.- Look for sequential scans on large tables to identify missing index opportunities.