Mastering Performance Troubleshooting with sys.dm_exec_requests in Azure SQL
Tags: azure, sql, sqlserver
Published: 2024-03-25 03:52:02

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.

```sql
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.

```sql
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`).

```sql
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.

```sql
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.

```sql
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.