Add info about slow query tables

src/SUMMARY.md

@@ -52,6 +52,9 @@
     * [Session](understand-tidb/session.md)
     * [Privilege](understand-tidb/privilege.md)
     * [Plugin](understand-tidb/plugin.md)
+    * [System tables](understand-tidb/system-tables/introduction.md)
+        * [information_schema](understand-tidb/system-tables/information_schema/introduction.md)
+            * [slow_query](understand-tidb/system-tables/information_schema/slow_query.md)
 
 * [Project Management](project-management/introduction.md)
     * [Releases Train Model](project-management/release-train-model.md)

src/system-tables/introduction.md

@@ -0,0 +1 @@
+# System tables

src/system-tables/slow_query.md

@@ -0,0 +1 @@
+# slow_query

src/understand-tidb/system-tables/information_schema/introduction.md

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+# information_schema
+
+* [slow_query](slow_query.md)

src/understand-tidb/system-tables/information_schema/slow_query.md

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+# slow_query
+
+## Overview
+
+Slow queries logs are written to `tidb-slow.log` (set with `slow-query-file` in the config) if they are over the `slow-threshold` (300ms by default). If `record-plan-in-slow-log` is enabled this will include the execution plan.
+
+Each TiDB node writes its own slow logs.
+
+### SLOW_QUERY
+```
+   +------------+
+   | App or CLI |
+   +------------+
+         |
+     SQL Query
+     SLOW_QUERY
+         |
+     +--------+
+     |  TiDB  |
+     +--------+
+         |
+  +---------------+
+  | tidb-slow.log |
+  +---------------+
+```
+
+The slow log can be viewed directly or via the `information_schema.SLOW_QUERY` table.
+
+The column definition is in [`slowQueryCols`](https://github.com/pingcap/tidb/blob/1521bf723dd023da655add0f883acaab5ee69683/pkg/infoschema/tables.go#L874-L956).
+
+The [`SlowQueryExtractor`](https://github.com/pingcap/tidb/blob/1521bf723dd023da655add0f883acaab5ee69683/pkg/planner/core/memtable_predicate_extractor.go#L1282) helps to extract some predicates of `slow_query`.
+
+A lot of the other logic can be found in [`slow_query.go`](https://github.com/pingcap/tidb/blob/master/pkg/executor/slow_query.go).
+
+This table has [`RuntimeStats`](https://github.com/pingcap/tidb/blob/2e51209f483bb7909be1eb0b55e5f18f0c437a25/pkg/executor/slow_query.go#L1114-L1119), which adds stats to `EXPLAIN ANALYZE` output.
+
+### CLUSTER_SLOW_QUERY
+
+There is also the `information_schema.CLUSTER_SLOW_QUERY` table that combines the slow log from all nodes into a single table.
+
+The [TiDB Dashboard](https://docs.pingcap.com/tidb/stable/dashboard-slow-query) uses the `CLUSTER_SLOW_QUERY` table to display the slow queries in a webpage.
+
+```
+                  +----------------+
+                  | TiDB Dashboard |
+                  |   or App/CLI   |
+                  +----------------+
+                         |
+                      SQL Query
+                  CLUSTER_SLOW_QUERY
+                         |
+                      +--------+
+        +--gRPC-------| TiDB 1 |------gRPC--+
+        |             +--------+            |
+   +--------+            |               +--------+
+   | TiDB 0 |            |               | TiDB 2 |
+   +--------+            |               +--------+
+      |                  |                  |
++---------------+  +---------------+  +---------------+
+| tidb-slow.log |  | tidb-slow.log |  | tidb-slow.log |
++---------------+  +---------------+  +---------------+
+```
+
+The protobuf messages are defined in [`diagnosticspb.proto`](https://github.com/pingcap/kvproto/blob/master/proto/diagnosticspb.proto) in the `pingcap/kvproto` repo. Key messages are `SearchLogRequest` and `SearchLogResponse`.
+
+This table uses the [`clusterLogRetriever`](https://github.com/pingcap/tidb/blob/1521bf723dd023da655add0f883acaab5ee69683/pkg/executor/memtable_reader.go#L361) with the [`ClusterLogTableExtractor`](https://github.com/pingcap/tidb/blob/1521bf723dd023da655add0f883acaab5ee69683/pkg/planner/core/memtable_predicate_extractor.go#L756).
+
+There are no `RuntimeStats` for this table, see [this issue](https://github.com/pingcap/tidb/issues/56707) for a request to add that.
+
+## Documentation
+
+- User docs
+    - [information_schema.slow_query](https://docs.pingcap.com/tidb/stable/information-schema-slow-query)
+    - [Identify Slow Queries](https://docs.pingcap.com/tidb/stable/identify-slow-queries#query-the-number-of-slow-queries-for-each-tidb-node-in-a-cluster)

src/understand-tidb/system-tables/introduction.md

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+# System tables
+
+There are multiple types of system tables:
+
+- `mysql` schema: There are mostly real tables stored on TiKV
+- [`information_schema`](information_schema/introduction.md): These tables are generated on request and not stored on disk.
+- `performance_schema`: These tables are generated on request and not stored on disk.
+- `metrics_schema`: These are metrics from Prometheus, not real tables.
+- `SYS`: these are views etc to make `performance_schema` easier to use.
+
+See also the docs:
+- [TiDB mysql schema](https://docs.pingcap.com/tidb/stable/mysql-schema)
+- [TiDB information_schema](https://docs.pingcap.com/tidb/stable/information-schema)
+- [TiDB performance_schema](https://docs.pingcap.com/tidb/stable/performance-schema)
+- [TiDB sys schema](https://docs.pingcap.com/tidb/stable/sys-schema)
+- [TiDB metrics_schema](https://docs.pingcap.com/tidb/stable/metrics-schema)
+- [MySQL performance_schema](https://dev.mysql.com/doc/refman/8.4/en/performance-schema.html)
+- [MySQL information_schema](https://dev.mysql.com/doc/refman/8.4/en/information-schema.html)
+- [MySQL sys schema](https://dev.mysql.com/doc/refman/8.4/en/sys-schema.html)
+- [MySQL mysql schema](https://dev.mysql.com/doc/refman/8.4/en/system-schema.html)