Mastering Performance Troubleshooting with sys.dm_exec_requests in Azure SQL

The Dynamic Management View (DMV) sys.dm_exec_requests is an invaluable tool in Azure SQL Database for troubleshooting performance issues. It provides real-time data about each request currently executing within SQL Server, including queries, background tasks, and system processes. Here's how to leverage sys.dm_exec_requests for performance troubleshooting:

1. Identify Long-Running Queries

You can use sys.dm_exec_requests to identify long-running queries that might be causing performance bottlenecks. By examining the start_time and total_elapsed_time columns, you can pinpoint queries that have been running for an unusually long time.

SELECT session_id, start_time, status, command, total_elapsed_time, text
FROM sys.dm_exec_requests
CROSS APPLY sys.dm_exec_sql_text(sql_handle)
WHERE status = 'running'
ORDER BY total_elapsed_time DESC;

2. Analyze Wait Types and Wait Times

sys.dm_exec_requests provides details on what the executing queries are waiting on through the wait_type and wait_time columns. This can help identify the types of resources that are causing delays.

SELECT session_id, wait_type, wait_time, blocking_session_id, text
FROM sys.dm_exec_requests
CROSS APPLY sys.dm_exec_sql_text(sql_handle)
WHERE wait_type IS NOT NULL
ORDER BY wait_time DESC;

3. Find Blocking Queries

Blocking occurs when one query holds a lock on a resource that another query needs. sys.dm_exec_requests can help identify blocking chains by showing which sessions are being blocked and which session is doing the blocking (blocking_session_id).

SELECT blocking_session_id, session_id, wait_type, wait_time, text
FROM sys.dm_exec_requests
CROSS APPLY sys.dm_exec_sql_text(sql_handle)
WHERE blocking_session_id <> 0
ORDER BY blocking_session_id;

4. Monitor Resource Usage

By observing the cpu_time, reads, writes, and logical_reads columns, you can get a sense of the resource usage of the currently executing requests. This can highlight queries that are particularly resource-intensive.

SELECT session_id, command, cpu_time, reads, writes, logical_reads, text
FROM sys.dm_exec_requests
CROSS APPLY sys.dm_exec_sql_text(sql_handle)
ORDER BY cpu_time DESC, reads DESC, logical_reads DESC;

5. Understand Query Execution Plans

To dive deeper into why a query is performing in a certain way, you can retrieve the execution plan for currently executing queries using the plan_handle column.

SELECT r.session_id, r.status, r.command, q.query_plan
FROM sys.dm_exec_requests r
CROSS APPLY sys.dm_exec_query_plan(r.plan_handle) q
WHERE r.session_id = @YourSessionId; -- Replace with the session ID of interest

Best Practices

• Regular Monitoring: Regularly monitor sys.dm_exec_requests during peak usage times to proactively identify and address potential performance issues.
• Combine with Other DMVs: Use sys.dm_exec_requests in conjunction with other Dynamic Management Views, like sys.dm_exec_sessions and sys.dm_exec_query_stats, for a more comprehensive performance analysis.
• Query Optimization: Based on insights from sys.dm_exec_requests, optimize long-running or resource-intensive queries through indexing, query rewriting, or schema changes.

Using sys.dm_exec_requests effectively requires a blend of real-time analysis and strategic follow-up actions to optimize performance and ensure smooth operation of Azure SQL Database environments.