Top 5 SQL Server Mistakes That Hurt Performance (And How to Fix Them)

SQL Server is a powerful engine—when used correctly. But even a well-configured server can grind to a halt due to avoidable missteps. 
If your queries are crawling, reporting is slow, or your application users are complaining, chances are you’re making one (or more) of the top SQL Server mistakes that are silently killing your performance. 
Let’s break them down—and show you how to fix them. 


1. Missing or Improper Indexing 
The Problem: SQL Server relies on indexes to retrieve data efficiently. Without the right indexes, even simple queries turn into full table scans—dragging your system down. 
Symptoms: 
● Slow SELECT queries 
● High CPU usage 
● Long report generation times 
The Fix: 
● Use SQL Server’s Execution Plan to identify missing indexes 
● Create clustered and non-clustered indexes strategically 
● Periodically rebuild and reorganize indexes to avoid fragmentation 
Bonus Tip: Don’t over-index either—it can slow down INSERT and UPDATE operations. 
 
2. Ignoring Query Optimization 
The Problem: Poorly written queries—even with the right data—can stall performance. Nested subqueries, missing WHERE clauses, and SELECT * are common culprits. 
Symptoms: 
● Long query execution time 
● Timeout errors 
● High IO operations 
The Fix: 
● Avoid SELECT *—always call only the needed columns 
● Replace nested subqueries with JOINs when possible 
● Use parameters and stored procedures for reusable, optimized logic 
Use SQL Profiler and Database Tuning Advisor to detect and tune problem queries. 
 
3. Not Archiving or Purging Old Data 
The Problem: A bloated database slows everything—from query performance to backup times. Many teams forget to clean up historical or obsolete data. 
Symptoms: 
● Database size grows rapidly 
● Slower performance on large tables 
● Backup/restore operations take hours 
The Fix: 
● Implement data retention policies 
● Archive old records to separate storage or tables 
● Schedule regular data purging jobs 
Remember: Not all data needs to be live in the primary transactional tables. 
 
4. Poor Configuration and Server Settings 
The Problem: Default settings are not one-size-fits-all. Improper memory allocation, parallelism settings, or tempdb mismanagement can throttle your system. 
Symptoms: 
● Server memory or CPU always maxed out 
● Performance lags during peak load 
● Frequent tempdb contention 
The Fix: 
● Tune max degree of parallelism (MAXDOP) based on your workload 
● Ensure tempdb has multiple data files and enough space 
● Set SQL Server memory limits to avoid OS starvation 
Work with an expert DBA to audit and optimize your SQL Server configuration. 
 
5. Neglecting Maintenance Tasks 
The Problem: Just like a car needs oil changes, SQL Server needs regular maintenance—backups, statistics updates, and index checks. 
Symptoms: 
● Inconsistent performance 
● Query plans become inefficient 
● Higher risk of data loss or corruption 
The Fix: 
● Automate index maintenance, statistics updates, and database integrity checks 
● Schedule full, differential, and transaction log backups 
● Use SQL Agent Jobs or tools like Ola Hallengren’s Maintenance Solution 
Set it and forget it—but monitor it regularly.