add planned commands folder

Signed-off-by: YANGDB <[email protected]>

docs/ppl-lang/PPL-Example-Commands.md

@@ -353,3 +353,16 @@ source = supplier
 
 > ppl-correlation-command is an experimental command - it may be removed in future versions
 
+---
+### Planned Commands:
+
+#### **fillnull**
+
+```sql
+   -  `source=accounts | fillnull fields status_code=101`
+   -  `source=accounts | fillnull fields request_path='/not_found', timestamp='*'`
+    - `source=accounts | fillnull using field1=101`
+    - `source=accounts | fillnull using field1=concat(field2, field3), field4=2*pi()*field5`
+    - `source=accounts | fillnull using field1=concat(field2, field3), field4=2*pi()*field5, field6 = 'N/A'`
+```
+[See additional command details](planning/ppl-fillnull-command.md)

docs/ppl-lang/README.md

@@ -89,6 +89,10 @@ For additional examples see the next [documentation](PPL-Example-Commands.md).
 See samples of [PPL queries](PPL-Example-Commands.md) 
 
 ---
+### Planned PPL Commands
 
+ - [`FillNull`](planning/ppl-fillnull-command.md)
+
+---
 ### PPL Project Roadmap
 [PPL Github Project Roadmap](https://github.com/orgs/opensearch-project/projects/214)

docs/ppl-lang/planning/ppl-fillnull-command.md

@@ -0,0 +1,50 @@
+## fillnull syntax proposal
+
+1. **Proposed syntax changes with `null` replacement with the same values in various fields**
+    - `... | fillnull with 0 in field1`
+    - `... | fillnull with 'N/A' in field1, field2, field3`
+    - `... | fillnull with 2*pi() + field1 in field2`
+    - `... | fillnull with concat(field1, field2) in field3, field4`
+    - `... | fillnull with 'N/A'`
+        - incorrect syntax
+    - `... | fillnull with 'N/A' in`
+        - validation error related to missing columns
+
+2. **Proposed syntax changes with `null` replacement with the various values in various fields**
+* currently implemented, not conform to previous syntax proposal (`fields` addition)
+    - `... | fillnull fields status_code=101`
+    -  `... | fillnull fields request_path='/not_found', timestamp='*'`
+* New syntax proposal
+    - `... | fillnull using field1=101`
+    - `... | fillnull using field1=concat(field2, field3), field4=2*pi()*field5`
+    - `... | fillnull using field1=concat(field2, field3), field4=2*pi()*field5, field6 = 'N/A'`
+    - `... | fillnull using`
+        -  validation error related to missing columns
+
+### New syntax definition in ANTLR
+
+```ANTLR
+  
+fillnullCommand
+   : FILLNULL (fillNullWithTheSameValue
+   | fillNullWithFieldVariousValues)
+   ;
+
+ fillNullWithTheSameValue
+ : WITH nullReplacement IN nullableField (COMMA nullableField)*
+ ;
+
+ fillNullWithFieldVariousValues
+ : USING nullableField EQUAL nullReplacement (COMMA nullableField EQUAL nullReplacement)*
+ ;
+
+
+   nullableField
+   : fieldExpression
+   ;
+
+   nullReplacement
+   : expression
+   ;
+
+```