docs: map type specific docs (#3886)

Author: aceforeverd

docs/en/openmldb_sql/data_types/composite_types.md

@@ -0,0 +1,30 @@
+# Composite data type
+
+## MAP type
+
+Represents values comprising a set of key-value pairs.
+
+**Syntax**
+
+```sql
+MAP < KEY, VALUE >
+```
+
+**Example**
+
+```sql
+-- construct map value with map builtin function, each parameter is 
+-- key1, value1, key2, value2, ... respectively
+select map (1, "12", 2, "100")
+-- {1: "12", 2: "100"}
+
+-- access map value with [] operator
+select map (1, "12", 2, "100")[2]
+-- "100"
+```
+
+**Limitations**
+
+1. Generally not recommended to store a map value with too much key-value pairs, since it's a row-based storage model.
+2. Map data type can not used as the key or ts column of table index, queries can not be optimized based on specific key value inside a map column neither.
+3. Query a key-value in a map takes `O(n)` complexity at most.

docs/en/openmldb_sql/data_types/index.rst

@@ -8,4 +8,5 @@ Data Type
 
     numeric_types
     string_types
+    composite_types
     date_and_time_types

docs/zh/openmldb_sql/data_types/composite_types.md

@@ -0,0 +1,29 @@
+# 复合数据类型
+
+## MAP 类型
+
+包含 key-value pair 的 map 类型。
+
+**Syntax**
+
+```sql
+MAP < KEY, VALUE >
+```
+
+**Example**
+
+```sql
+-- 使用 map function 构造 map 数据类型，参数 1-N 表示 key1, value1, key2, value2, ...
+select map (1, "12", 2, "100")
+-- {1: "12", 2: "100"}
+
+-- 用 [] operator 抽取特定 key 的 value
+select map (1, "12", 2, "100")[2]
+-- "100"
+```
+
+**限制**
+
+1. 由于采用行存储形式，不建议表的 MAP 类型存储 key-value pair 特别多的情况，否则可能导致性能问题。
+2. map 数据类型不支持作为索引的 key 或 ts 列，无法对 map 列特定 key 做查询优化。 
+3. map key-value 查询最多消耗 `O(n)` 复杂度

docs/zh/openmldb_sql/data_types/index.rst

@@ -9,3 +9,4 @@
     numeric_types
     string_types
     date_and_time_types
+    composite_types