Added design for core refactor

Signed-off-by: Guian Gumpac <guian.gumpac@improving.com>

docs/dev/core-refactor.md

@@ -0,0 +1,71 @@
+# 1. Overview
+
+Improve extensibility of the SQL plugin, OpenSearch functions need to be moved from `core` module and moved to the `opensearch` module. This will prevent other data sources from being able to parse queries with OpenSearch specific functions like `MATCH_QUERY`.
+
+# 2. Requirements
+
+Resolve https://github.com/opensearch-project/sql/issues/811
+
+## 2.1 Use case
+
+1. Using SQL or PPL with OpenSearch data source. Users should be able to use all available functions that is supported by SQL, PPL, and OpenSearch.
+2. Using SQL or PPL with data sources other than OpenSearch (Prometheus, for example). Users should be able to use implemented SQL and PPL functions.
+
+## 2.2 Functional change
+
+- Using data sources other than OpenSearch, users will encounter a `SyntaxCheckException` when they query with OpenSearch specific functions. (Some of these functions can be found in https://opensearch.org/docs/latest/search-plugins/sql/sql/functions/)
+- The Parser will have a different token to categorize OpenSearch functions.
+- Core module will no longer contain OpenSearch functions.
+
+# 3. Design
+
+## 3.1 Datasource specific parser
+
+Core will maintain its parser, however OpenSearch functions will be under a new token called `userDefinedFunctions`. This new token will encompass datasource specific functions. Each datasource module will have its own parser which will be called once the core module analyzer visits this `userDefinedFunction` node in the generated parse tree. Then, the datasource module will parse this function and with its own analyzer, add these function nodes to the parse tree.
+
+```mermaid
+sequenceDiagram
+    participant Core Parser
+    participant Core Analyzer
+    participant #8249;DataSource#8250; Parser
+    participant #8249;DataSource#8250; Analyzer
+    participant QueryPlanFactory
+    
+    Core Parser ->> Core Analyzer: #8249;DataSource#8250; specific query
+    Core Analyzer ->> #8249;DataSource#8250; Parser: #8249;DataSource#8250; specific query
+    #8249;DataSource#8250; Parser ->> #8249;DataSource#8250; Analyzer: #8249;DataSource#8250; specific query
+    #8249;DataSource#8250; Analyzer ->> #8249;DataSource#8250; Parser: ParserTree
+    #8249;DataSource#8250; Parser ->> Core Analyzer: ParserTree
+    Core Analyzer ->> Core Parser: ParserTree
+    Core Parser ->> QueryPlanFactory: ParserTree
+```
+
+```mermaid
+flowchart LR
+    A[User] -- query --> B[Core Parser]
+    B -- query --> C[Core Analyzer]
+    C --> D{DS specific?}
+    D --> |True| E[DS Parser]
+    E -- query --> F[DS Analyzer]
+    F -- ParserTree --> G[QueryPlanFactory]
+    D --> |False\nParserTree| G
+```
+
+If the function for the above is `MATCH_QUERY`, OpenSearch will be the DataSource.
+
+## 3.2 Datasource specific LogicalPlanOptimizer
+
+ Rules in core LogicalPlanOptimizer should not include Datasource specific push down rules. For example, PUSH_DOWN_NESTED will be removed from the core LogicalPlanOptimizer. When optimizing a list of rules will be requested from the datasource module (This list can be empty. Prometheus would return an empty list currently).
+```mermaid
+sequenceDiagram
+    participant Planner
+    participant Core LogicalPlanOptimizer
+    participant #8249;DataSource#8250; LogicalPlanOptimizer
+    
+    Planner ->> Core LogicalPlanOptimizer:  create()
+    Core LogicalPlanOptimizer -->> Planner: general rules list
+    Planner ->> Core LogicalPlanOptimizer:   optimize()
+    Core LogicalPlanOptimizer ->> #8249;DataSource#8250; LogicalPlanOptimizer: getList()
+    #8249;DataSource#8250; LogicalPlanOptimizer -->> Core LogicalPlanOptimizer: Datasource specific rules list
+    Core LogicalPlanOptimizer -->> Planner: LogicalPlan
+```