MSDN Documentation

Introduction

Performance issues in SQL Server can manifest in various ways, from slow query execution to unresponsive applications. This guide provides a systematic approach to identify, diagnose, and resolve common performance bottlenecks.

Effective troubleshooting requires a deep understanding of SQL Server's architecture, resource utilization, and query execution plans. We will cover key areas such as CPU, memory, I/O, locking, and query optimization.

CPU Issues

High CPU utilization can significantly degrade performance. Common causes include inefficient queries, excessive compilation, and background processes.

Diagnosis

• Monitor CPU usage using Task Manager or Performance Monitor (PerfMon).
• Identify processes consuming the most CPU, particularly the sqlservr.exe process.
• Use Dynamic Management Views (DMVs) like sys.dm_os_wait_stats and sys.dm_exec_requests to pinpoint resource-intensive queries or processes.

Resolution

• Optimize poorly performing queries (see Query Performance section).
• Reduce query recompilations by using stored procedures and parameter sniffing best practices.
• Check for blocking or long-running transactions.
• Ensure SQL Server is configured with an appropriate number of costed worker threads.

Example T-SQL Query (Top CPU Queries)

SELECT TOP 50
    qs.total_cpu_time,
    qs.total_elapsed_time,
    SUBSTRING(st.text, (qs.statement_start_offset/2)+1,
        ((CASE qs.statement_end_offset
            WHEN -1 THEN DATALENGTH(st.text)
            ELSE qs.statement_end_offset
         END - qs.statement_start_offset)/2) + 1) AS statement_text,
    qp.query_plan
FROM sys.dm_exec_query_stats AS qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp
ORDER BY qs.total_cpu_time DESC;
                

Memory Issues

Insufficient memory can lead to excessive paging, impacting overall system performance.

Diagnosis

• Monitor memory usage in Task Manager/PerfMon. Pay attention to page faults per second.
• Use sys.dm_os_performance_counters to check SQLServer:Buffer Manager\Page life expectancy and SQLServer:Memory Manager\Target Server Memory (KB).
• Check for memory leaks or excessive memory grants.

Resolution

• Ensure SQL Server has adequate memory allocated and that the "Max Server Memory" setting is configured appropriately.
• Optimize queries to reduce memory consumption (e.g., avoid large sorts or temporary tables).
• Identify and address inefficient indexing that might lead to table scans and higher memory usage.
• Consider increasing physical RAM if the server is consistently starved for memory.

I/O Bottlenecks

Slow disk performance is a common bottleneck, affecting data retrieval and writes.

Diagnosis

• Monitor disk queue length and average disk sec/read or sec/write in PerfMon.
• Use sys.dm_io_virtual_file_stats to analyze I/O latency for each database file.
• Look for high values in PAGEIOLATCH_* wait types in sys.dm_os_wait_stats.

Resolution

• Ensure storage subsystem is adequate for the workload.
• Optimize queries to reduce the amount of data read from disk (e.g., use covering indexes).
• Distribute database files (data, log, tempdb) across different physical disks or volumes.
• Consider using faster storage solutions (e.g., SSDs).
• Regularly maintain indexes and statistics.

Locking & Deadlocks

Excessive locking and deadlocks can block transactions and applications.

Diagnosis

• Monitor sys.dm_os_waiting_tasks for tasks waiting on lock resources (LCK_* waits).
• Analyze deadlock graphs by setting up a SQL Server trace or using Extended Events to capture deadlock information.
• Identify blocking sessions using sp_who2 or sys.dm_exec_requests and sys.dm_tran_locks.

Resolution

Best Practices

• Keep transactions short and concise.
• Access objects in the same order across different transactions.
• Use appropriate isolation levels.
• Optimize queries to reduce the duration of locks held.
• Ensure efficient indexing to minimize the number of rows scanned.
• Consider using the READ_COMMITTED_SNAPSHOT or SNAPSHOT isolation levels if applicable.

Query Performance

Inefficient queries are a primary cause of performance problems.

Diagnosis

• Identify slow queries using SQL Server Profiler, Extended Events, or DMVs like sys.dm_exec_query_stats.
• Analyze execution plans for queries, looking for table scans, index scans, key lookups, and costly operators.
• Check missing or outdated statistics.

Resolution

• Create appropriate indexes to support query predicates and joins.
• Rewrite queries to be more efficient, avoiding `SELECT *`, unnecessary joins, and correlated subqueries where possible.
• Ensure statistics are up-to-date using UPDATE STATISTICS.
• Consider using query hints judiciously if necessary, but prioritize query and index optimization.
• Use parameterized queries and stored procedures to improve plan caching.

Key Execution Plan Operators to Watch

• Table Scan/Clustered Index Scan: Indicates a full scan of a table or index.
• Key Lookup: Occurs when a non-clustered index is used, and subsequent rows need to be retrieved from the clustered index.
• Sort: Can be expensive, especially on large datasets.
• Hash Match: Used for joins and aggregations; can be memory-intensive.
• Nested Loops: Efficient for small outer inputs; can be slow for large inputs.

Tools and Resources

• SQL Server Management Studio (SSMS): Provides graphical execution plans, execution statistics, and query analysis tools.
• SQL Server Profiler: A tool for tracing and analyzing SQL Server events.
• Extended Events: A flexible and lightweight tracing system.
• Dynamic Management Views (DMVs): Provide real-time server state information.
• Performance Monitor (PerfMon): For system-level performance counters.