Lära Troubleshooting Slow Queries with EXPLAIN | Real-World Query Optimization Scenarios

When you encounter slow queries in a production-like environment, the first step is to identify the root cause. Common causes of slow queries include missing or inefficient indexes, unnecessary full table scans, poorly written joins, suboptimal filtering, and returning more data than necessary. The SQL EXPLAIN command is a powerful tool to help you diagnose these issues by showing how the database executes your query. By inspecting the query plan, you can see whether indexes are being used, how tables are joined, what type of scans are performed, and which steps are the most expensive in terms of cost and row estimates.


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EXPLAIN
SELECT r.first_name, r.last_name, ra.rating, p.amount, p.payment_time
FROM riders r
JOIN ratings ra ON r.rider_id = ra.rider_id
JOIN payments p ON r.rider_id = p.rider_id
WHERE ra.rating >= 4
ORDER BY p.payment_time DESC;

To troubleshoot a slow query using the EXPLAIN output, start by reading the plan from the bottom up. Look for steps that involve sequential scans (Seq Scan) on large tables, which often indicate missing indexes. Notice if hash joins or nested loops are used where a more efficient join could be possible with proper indexing. Pay attention to the estimated cost and row counts for each operation; high values may point to bottlenecks. If the plan shows multiple scans or redundant joins, consider rewriting the query for efficiency. Evaluate the filtering and ordering steps, as sorting large result sets or filtering without indexes can be expensive. By following this step-by-step approach, you can pinpoint the exact part of the query that needs optimization.


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-- Optimized query with explicit indexes and selective filtering
EXPLAIN
SELECT r.first_name, r.last_name, ra.rating, p.amount, p.payment_time
FROM riders r
JOIN ratings ra ON r.rider_id = ra.rider_id
JOIN payments p ON r.rider_id = p.rider_id
WHERE ra.rating >= 4
  AND p.payment_time > '2023-05-01'
ORDER BY p.payment_time DESC;

1. What are some common red flags in EXPLAIN output for slow queries?

2. How can you use EXPLAIN to decide where to add indexes?

3. Fill in the blank: A high value in the ______ field may indicate a performance issue.

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Suggested prompts:

Can you explain the differences between the two EXPLAIN outputs?

What indexes should I create to optimize this query further?

How do I interpret the cost and row estimates in the EXPLAIN plan?

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