[docs] Improve the doc structure of Merge Engines (#241)

Author: wuchong

…-design/table-types/pk-table/overview.md …ble-design/table-types/pk-table/index.mdwebsite/docs/table-design/table-types/pk-table/overview.md renamed to website/docs/table-design/table-types/pk-table/index.md website/docs/table-design/table-types/pk-table/overview.md renamed to website/docs/table-design/table-types/pk-table/index.md

@@ -2,7 +2,7 @@
 sidebar_position: 1
 ---
 
-# Overview
+# PrimaryKey Table
 
 ## Basic Concept
 
@@ -22,8 +22,8 @@ CREATE TABLE pk_table
     total_amount INT,
     PRIMARY KEY (shop_id, user_id) NOT ENFORCED
 ) WITH (
-      'bucket.num' = '4'
-      );
+    'bucket.num' = '4'
+);
 ```
 
 In Fluss primary key table, each row of data has a unique primary key.
@@ -73,6 +73,18 @@ follows:
 | 1 | 2.0 | t1 |
 | 2 | 3.0 | t2 |
 
+## Merge Engines
+
+The **Merge Engine** in Fluss is a core component designed to efficiently handle and consolidate data updates for PrimaryKey Tables.
+It offers users the flexibility to define how incoming data records are merged with existing records sharing the same primary key.
+The default merge engine in Fluss retains the latest record for a given primary key.
+However, users can specify a different merge engine to customize the merging behavior according to their specific use cases
+
+The following merge engines are supported:
+
+1. [FirstRow Merge Engine](/docs/table-design/table-types/pk-table/merge-engines/first-row)
+2. [Versioned Merge Engine](/docs/table-design/table-types/pk-table/merge-engines/versioned)
+
 ## Data Queries
 
 For primary key tables, Fluss supports querying data directly based on the key. Please refer to

website/docs/table-design/table-types/pk-table/merge-engine/_category_.json

@@ -0,0 +1,4 @@
+{
+  "label": "Merge Engines",
+  "position": 2
+}

website/docs/table-design/table-types/pk-table/merge-engine/first-row.md

@@ -0,0 +1,46 @@
+---
+sidebar_label: FirstRow
+sidebar_position: 2
+---
+
+# FirstRow Merge Engine
+
+By setting `'table.merge-engine' = 'first_row'` in the table properties, users can retain the first record for each primary key.
+This configuration generates an insert-only changelog, allowing downstream Flink jobs to treat the PrimaryKey Table as an append-only Log Table.
+As a result, downstream transformations that do not support retractions/changelogs, such as [Window Aggregations](https://nightlies.apache.org/flink/flink-docs-release-1.20/docs/dev/table/sql/queries/window-agg/)
+and [Interval Joins](https://nightlies.apache.org/flink/flink-docs-release-1.20/docs/dev/table/sql/queries/joins/#interval-joins), can be applied seamlessly.
+
+This feature is particularly valuable for replacing log deduplication in streaming computations, reducing complexity and improving overall efficiency.
+
+:::note
+When using `first_row` merge engine, there are the following limits:
+
+- `UPDATE` and `DELETE` SQL statements are not supported
+- Partial Update is not supported
+- `UPDATE_BEFORE` and `DELETE` changelog events are ignored automatically
+:::
+
+## Example
+
+```sql title="Flink SQL"
+CREATE TABLE T (
+    k  INT,
+    v1 DOUBLE,
+    v2 STRING,
+    PRIMARY KEY (k) NOT ENFORCED
+) WITH (
+    'table.merge-engine' = 'first_row'
+);
+
+INSERT INTO T VALUES (1, 2.0, 't1');
+INSERT INTO T VALUES (1, 3.0, 't2');
+
+SELECT * FROM T;
+
+-- Output
+-- +---+-----+------+
+-- | k | v1  | v2   |
+-- +---+-----+------+
+-- | 1 | 2.0 | t1   |
+-- +---+-----+------+
+```

website/docs/table-design/table-types/pk-table/merge-engine/overview.md

@@ -0,0 +1,15 @@
+---
+sidebar_position: 1
+---
+
+# Merge Engines
+
+The **Merge Engine** in Fluss is a core component designed to efficiently handle and consolidate data updates for PrimaryKey Tables.
+It offers users the flexibility to define how incoming data records are merged with existing records sharing the same primary key.
+The default merge engine in Fluss retains the latest record for a given primary key.
+However, users can specify a different merge engine to customize the merging behavior according to their specific use cases
+
+The following merge engines are supported:
+
+1. [FirstRow Merge Engine](/docs/table-design/table-types/pk-table/merge-engines/first-row)
+2. [Versioned Merge Engine](/docs/table-design/table-types/pk-table/merge-engines/versioned)

website/docs/table-design/table-types/pk-table/merge-engines/_category_.json

@@ -3,6 +3,6 @@ sidebar_label: Versioned
 sidebar_position: 3
 ---
 
-# Versioned
+# Versioned Merge Engine
 
 TODO: Fill me #459