Your SQL Query Works. Here’s Why It’s Still Slow.

Suhana Saini

3 min read

·

Oct 15, 2026

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A query that returns the right answer slowly is still a problem. Here are the most common reasons SQL queries underperform — and the fixes that actually help.

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1. SELECT * Is Pulling Data You Don’t Need

Pulls every column across every row:

SELECT * FROM orders JOIN customers ON orders.customer_id = customers.id;

If you only need three columns, pulling thirty costs you memory, network transfer, and in columnar storage systems (like Databricks or BigQuery), unnecessary column scans.

Fix: Name your columns explicitly. Yes, it’s more typing, but it’s faster and more readable.

SELECT 
    orders.id AS order_id, 
    orders.order_date, 
    orders.total_amount, 
    customers.id AS customer_id, 
    customers.first_name, 
    customers.last_name, 
    customers.email
FROM orders 
JOIN customers ON orders.customer_id = customers.id;

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2. You’re Filtering After Aggregating Instead of Before

— Slow: aggregates everything, then filters

SELECT customer_id, SUM(amount) 
FROM orders 
GROUP BY customer_id 
HAVING customer_id > 1000;

— Fast: filters first, then aggregates over smaller dataset