docs: json datatype rfc

docs/rfcs/2024-08-06-json-datatype.md

@@ -0,0 +1,63 @@
+---
+Feature Name: Json Datatype
+Tracking Issue: https://github.com/GreptimeTeam/greptimedb/issues/4230
+Date: 2024-8-6
+Author: "Yuhan Wang <[email protected]>"
+---
+
+# Summary
+This RFC proposes a method for storing and querying JSON data in the database.
+
+# Motivation
+JSON is widely used across various scenarios. Direct support for writing and querying JSON can significantly enhance the database's flexibility.
+
+# Details
+
+## User Interface
+The feature introduces a new data type for the database, similar to the common JSON type. Data is written as JSON strings and can be queried using functions.
+
+For example:
+```SQL
+CREATE TABLE IF NOT EXISTS test (
+    ts TIMESTAMP TIME INDEX,
+    a INT,
+    b JSON
+);
+
+INSERT INTO test VALUES(
+    0,
+    0,
+    '{
+        "name": "jHl2oDDnPc1i2OzlP5Y",
+        "timestamp": "2024-07-25T04:33:11.369386Z",
+        "attributes": { "event_attributes": 48.28667 }
+    }'
+);
+
+SELECT json_get(b, 'name') FROM test;
++---------------------+
+| b.name              |
++---------------------+
+| jHl2oDDnPc1i2OzlP5Y |
++---------------------+
+
+SELECT json_get_by_paths(b, 'attributes', 'event_attributes') + 1 FROM test;
++-------------------------------+
+| b.attributes.event_attributes |
++-------------------------------+
+|                      49.28667 |
++-------------------------------+
+
+```
+
+## Storage and Querying
+
+Data of JSON type is stored as JSONB format in the database. For storage layer, data is represented as a binary array and can be queried through pre-defined JSON functions. For clients, data is shown as strings and can be casted to other types if needed.
+
+# Drawbacks
+
+As a general purpose data type, JSONB may not be as efficient as specialized data types for specific scenarios.
+
+# Alternatives
+
+Extract and flatten JSON schema to store in a structured format through pipeline. For nested data, we can provide nested types like `STRUCT` or `ARRAY`.