feat: document query plans (#81)

Author: levkk

docs/enterprise_edition/index.md

@@ -16,7 +16,7 @@ The following features are available exclusively in the Enterprise edition:
 |-|-|
 | [Control plane](control_plane/index.md) (beta) | Manage multiple PgDog nodes and deployments. |
 | Queries | Monitor queries running through PgDog in real-time. |
-| Plans | Request and track Postgres query plans for slow queries. |
+| [Plans](plans.md) | Request and track Postgres query plans for slow queries. |
 | Metrics | Second-precision PgDog and resource usage metrics. |
 | [Quality of Service](qos.md) (alpha) | Track and block bad queries automatically. |
 
@@ -69,12 +69,7 @@ image:
 
 #### Control plane
 
-The [control plane](control_plane/index.md) comes with its own [Helm chart](control_plane/installation.md). The chart has a few cluster dependencies, which you can check using our installation script:
-
-```bash
-curl -fsSL \
-  https://raw.githubusercontent.com/pgdogdev/helm-ee/main/install.sh | bash
-```
+The [control plane](control_plane/index.md) comes with its own [Helm chart](control_plane/installation.md).
 
 ### From source
 

docs/enterprise_edition/plans.md

@@ -0,0 +1,83 @@
+---
+icon: material/file-tree
+---
+
+# Query plans
+
+Slow queries are difficult to debug in production. Postgres can generate a query plan for a specific statement, but the exact parameters that caused a bad plan are often unavailable.
+
+PgDog can automatically request and store query plans for slow queries as it serves them. Because plans are fetched in real time, they are more likely to capture the bad execution plan before Postgres autovacuum changes the underlying statistics.
+
+## How it works
+
+Query plan collection is disabled by default and can be enabled in `pgdog.toml`:
+
+=== "pgdog.toml"
+    ```toml
+    [query_stats]
+    enabled = true
+    query_plan_threshold = 100 # Queries slower than 100ms will be planned.
+    ```
+=== "Helm chart"
+    ```yaml
+    queryStats:
+      enabled: true
+      queryPlanThreshold: 100
+    ```
+
+The plans are stored in memory and deduplicated by _normalized_ SQL of the query. This means that queries with different parameters will be tied to the same query plan, making sure PgDog doesn't plan the same query multiple times.
+
+### Configuration
+
+The query plan cache is configurable:
+
+| Argument | Description | Example |
+|-|-|-|
+| `query_plan_threshold` | Minimum query runtime that triggers a query plan (in ms). | `100` |
+| `query_plans_sample_rate` | Percentage of all queries executed through PgDog that will be planned, irrespective of their runtime (between 0.0 and 1.0). | `0.5` |
+| `query_plans_cache` | Maximum number of entries in the in-memory plan cache. | `500` |
+| `query_plan_max_age` | Query plans older than this are considered stale and will be replanned if the query is executed again (in ms). | `15_000` |
+
+##### Example
+
+=== "pgdog.toml"
+    ```toml
+    [query_stats]
+    enabled = true
+    query_plan_threshold = 100
+    query_plans_sample_rate = 0.5
+    query_plans_cache = 1_000
+    query_plan_max_age = 15_000
+    ```
+=== "Helm chart"
+    ```yaml
+    queryStats:
+      enabled: true
+      queryPlanThreshold: 100
+      queryPlansSampleRate: 0.5
+      queryPlansCache: 1000
+      queryPlanMaxAge: 15000
+    ```
+
+### Admin database
+
+The query plans are viewable by querying the [admin database](../administration/index.md) with the `SHOW QUERY_PLANS` command:
+
+=== "Command"
+    ```
+    SHOW QUERY_PLANS;
+    ```
+=== "Output"
+    ```
+            query         |                   plan                   | database | user  |  age  |  reason
+    ----------------------+------------------------------------------+----------+-------+-------+-----------
+     select pg_sleep($1); | Result  (cost=0.00..0.01 rows=1 width=4) | pgdog    | pgdog | 48810 | threshold
+    ```
+
+### Control plane
+
+The query plans are joined with live queries and are sent to the [control plane](control_plane/index.md). They are viewable in the control plane UI and are updated in real time:
+
+<center>
+    <img src="/images/ee/plans.png" width="100%" alt="Control plane">
+</center>