roadmap: update TiDB roadmap (#654)

* roadmap: update TiDB roadmap

Via: pingcap/docs-cn#912

* roadmap: address the comment

Author: lilin90

ROADMAP.md

@@ -12,72 +12,68 @@ This document defines the roadmap for TiDB development.
 
 + [ ] Optimizer
     - [x] Refactor Ranger
-    - [ ] Optimize the cost model
+    - [x] Optimize the cost model
+    - [ ] Cascades model planner
     - [ ] Join Reorder
 + [ ] Statistics
     - [x] Update statistics dynamically according to the query feedback
     - [x] Analyze table automatically
-    - [ ] Improve the accuracy of Row Count estimation
-+ [ ] Executor
+    - [x] Improve the accuracy of Row Count estimation
++ [ ] Execution Engine
     - [ ] Push down the Projection operator to the Coprocessor
-    - [ ] Improve the performance of the HashJoin operator
-    - [ ] Parallel Operators
+    - [x] Improve the performance of the HashJoin operator
+    + [ ] Parallel Operators
         - [x] Projection
-        - [ ] Aggregation
+        - [x] Aggregation
         - [ ] Sort
     - [x] Compact Row Format to reduce memory usage
     - [ ] File Sort
 - [ ] View
 - [ ] Window Function
 - [ ] Common Table Expression
-- [ ] Table Partition
++ [ ] Table Partition
+    - [x] Range Partition
+    - [ ] Hash Partition
 - [ ] Cluster Index
-- [ ] Improve DDL
-  - [x] Speed up Add Index operation
-  - [ ] Parallel DDL
+- [ ] New storage row format
+- [ ] Query Tracing
++ [ ] Improve DDL
+    - [x] Speed up Add Index operation
+    - [x] Parallel DDL
+    - [ ] Support locking table
+    - [ ] Support modifying the column type
+    - [ ] Supoort modifying the primary key
+    - [ ] Support multiple DDL operations in a single statement
 - [ ] Support `utf8_general_ci` collation
 
 ## TiKV:
 
-### Raft
-
-- [x] Region Merge - Merge small Regions together to reduce overhead
-- [x] Local Read Thread - Process read requests in a local read thread
-- [x] Split Region in Batch - Speed up Region split for large Regions
-- [x] Raft Learner - Support Raft learner to smooth the configuration change process
-- [x] Raft Pre-vote - Support Raft pre-vote to avoid unnecessary leader election on network isolation
-- [ ] Joint Consensus - Change multi members safely.
-- [ ] Multi-thread Raftstore - Process Region Raft logic in multiple threads
-- [ ] Multi-thread apply pool - Apply Region Raft committed entries in multiple threads
-
-### Engine
-
-- [ ] Titan - Separate large key-values from LSM-Tree
-- [ ] Pluggable Engine Interface - Clean up the engine wrapper code and provide more extendibility
-
-### Storage
-
-- [ ] Flow Control - Do flow control in scheduler to avoid write stall in advance
-
-### Transaction
-
-- [x] Optimize transaction conflicts
-- [ ] Distributed GC - Distribute MVCC garbage collection control to TiKV
-
-### Coprocessor
-
-- [x] Streaming - Cut large data set into small chunks to optimize memory consumption
-- [ ] Chunk Execution - Process data in chunk to improve performance
-- [ ] Request Tracing - Provide per-request execution details
-
-### Tools
-
-- [x] TiKV Importer - Speed up data importing by SST file ingestion
-
-### Client
-
-- [ ] TiKV client (Rust crate)
-- [ ] Batch gRPC Message - Reduce message overhead
++ Raft
+    - [x] Region Merge - Merge small Regions together to reduce overhead
+    - [x] Local Read Thread - Process read requests in a local read thread
+    - [x] Split Region in Batch - Speed up Region split for large Regions
+    - [x] Raft Learner - Support Raft learner to smooth the configuration change process
+    - [x] Raft Pre-vote - Support Raft pre-vote to avoid unnecessary leader election on network isolation
+    - [ ] Joint Consensus - Change multi members safely.
+    - [ ] Multi-thread Raftstore - Process Region Raft logic in multiple threads
+    - [ ] Multi-thread apply pool - Apply Region Raft committed entries in multiple threads
++ Engine
+    - [ ] Titan - Separate large key-values from LSM-Tree
+    - [ ] Pluggable Engine Interface - Clean up the engine wrapper code and provide more extensibility
++ Storage
+    - [ ] Flow Control - Do flow control in scheduler to avoid write stall in advance
++ Transaction
+    - [x] Optimize transaction conflicts
+    - [ ] Distributed GC - Distribute MVCC garbage collection control to TiKV
++ Coprocessor
+    - [x] Streaming - Cut large data set into small chunks to optimize memory consumption
+    - [ ] Chunk Execution - Process data in chunk to improve performance
+    - [ ] Request Tracing - Provide per-request execution details
++ Tools
+    - [x] TiKV Importer - Speed up data importing by SST file ingestion
++ Client
+    - [ ] TiKV client (Rust crate)
+    - [ ] Batch gRPC Message - Reduce message overhead
 
 ## PD: