Add Top SQL doc pingcap#8123 (pingcap#7418)

TOC.md

@@ -91,7 +91,8 @@
   - [Identify Slow Queries](/identify-slow-queries.md)
   - [Analyze Slow Queries](/analyze-slow-queries.md)
   - [SQL Diagnostics](/information-schema/information-schema-sql-diagnostics.md)
-  - [Identify Expensive Queries](/identify-expensive-queries.md)
+  - [Identify Expensive Queries Using Top SQL](/dashboard/top-sql.md)
+  - [Identify Expensive Queries Using Logs](/identify-expensive-queries.md)
   - [Statement Summary Tables](/statement-summary-tables.md)
   - [Troubleshoot Hotspot Issues](/troubleshoot-hot-spot-issues.md)
   - [Troubleshoot Increased Read and Write Latency](/troubleshoot-cpu-issues.md)
@@ -619,6 +620,7 @@
       - [Access](/dashboard/dashboard-access.md)
       - [Overview Page](/dashboard/dashboard-overview.md)
       - [Cluster Info Page](/dashboard/dashboard-cluster-info.md)
+      - [Top SQL](/dashboard/top-sql.md)
       - [Key Visualizer Page](/dashboard/dashboard-key-visualizer.md)
       - [Metrics Relation Graph](/dashboard/dashboard-metrics-relation.md)
       - SQL Statements Analysis

dashboard/top-sql.md

@@ -0,0 +1,43 @@
+---
+title: Top SQL
+summary: This document describes how to use Top SQL to locate SQL queries that contribute to a high load.
+---
+
+# Top SQL
+
+> **Warning:**
+>
+> Currently, Top SQL is an experimental feature. It is not recommended that you use it for production environments.
+
+This document describes how to use Top SQL to locate SQL queries that contribute to a high load of a TiDB or TiKV node in a specified time range. For example, you can use Top SQL to locate an analytic query that consumes 99% of the load for a low-load database.
+
+For a specified TiDB or TiKV node, Top SQL provides the following features:
+
+* Show the top 5 types of SQL queries that contribute the most to the load in a specified time range.
+* Show information such as CPU usage, requests per second, average latency, and query plan of a particular query, which can be used for potential performance optimization to improve your business.
+
+## Enable Top SQL
+
+The Top SQL feature is disabled by default. You can enable the feature for the entire cluster using either of the following methods:
+
+- Method 1: Log in to TiDB Dashboard, click **Top SQL** in the left pane, click the gear button in the upper-right corner of the page, and then enable the Top SQL feature.
+- Method 2: Set the value of the TiDB system variable [`tidb_enable_top_sql`](/system-variables.md#tidb_enable_top_sql-new-in-v540) to `ON`.
+
+> **Note:**
+>
+> Enabling Top SQL has a slight impact on the performance of your cluster.
+
+## Use Top SQL
+
+Once Top SQL is enabled, you can log into TiDB Dashboard, and then click **Top SQL** in the left pane to use it.
+
+![Top SQL](/media/dashboard/top-sql-overview.png)
+
+Usage tips：
+
+* You can select the target node and time range in the drop-down lists at the top of the page, or you can select the time range in the chart.
+* If the data in the chart is out of date, you can click **Refresh**, or select auto-refresh and specify the auto-refresh interval in the **Refresh** drop-down list.
+* The chart shows the top 5 types of queries that contribute the most to the load of the selected node in the selected time range.
+* You can select a query type in the list to view the execution plan of that query type on this node and the execution details such as the Call/sec, Scan Rows/sec, Scan Indexes/sec, and Latency/call.
+
+![Top SQL Details](/media/dashboard/top-sql-details.png)

system-variables.md

@@ -1585,6 +1585,16 @@ SET tidb_slow_log_threshold = 200;
 - Unit: Seconds
 - This variable is used to set the refresh time of [statement summary tables](/statement-summary-tables.md).
 
+### `tidb_enable_top_sql` <span class="version-mark">New in v5.4.0</span>
+
+- Scope: GLOBAL
+- Default value: `OFF`
+- This variable is used to control whether to enable the [Top SQL](/dashboard/top-sql.md) feature.
+
+> **Warning:**
+>
+> Currently, Top SQL is an experimental feature. It is not recommended that you use it for production environments.
+
 ### tidb_store_limit <span class="version-mark">New in v3.0.4 and v4.0</span>
 
 - Scope: GLOBAL