add changelog for v2.1.0-rc.5 (#8272) (#8275)

Author: winkyao

CHANGELOG.md

@@ -1,6 +1,38 @@
 # TiDB Changelog
 All notable changes to this project will be documented in this file. See also [Release Notes](https://github.com/pingcap/docs/blob/master/releases/rn.md), [TiKV Changelog](https://github.com/tikv/tikv/blob/master/CHANGELOG.md) and [PD Changelog](https://github.com/pingcap/pd/blob/master/CHANGELOG.md).
 
+## [2.1.0-rc.5] - 2018-11-12
+### SQL Optimizer
+* Fix the issue that `IndexReader` reads the wrong handle in some cases [#8132](https://github.com/pingcap/tidb/pull/8132)
+* Fix the issue occurred while the `IndexScan Prepared` statement uses `Plan Cache` [#8055](https://github.com/pingcap/tidb/pull/8055)
+* Fix the issue that the result of the `Union` statement is unstable [#8165](https://github.com/pingcap/tidb/pull/8165)
+### SQL Execution Engine
+* Improve the performance of TiDB on inserting or updating wide tables [#8024](https://github.com/pingcap/tidb/pull/8024)
+* Support the unsigned `int` flag in the `Truncate` built-in function [#8068](https://github.com/pingcap/tidb/pull/8068)
+* Fix the error occurred while converting JSON data to the decimal type [#8109](https://github.com/pingcap/tidb/pull/8109)
+* Fix the error occurred when you `Update` the float type [#8170](https://github.com/pingcap/tidb/pull/8170)
+### Statistics
+* Fix the incorrect statistics issue during point queries in some cases [#8035](https://github.com/pingcap/tidb/pull/8035)
+* Fix the selectivity estimation of statistics for primary key in some cases [#8149](https://github.com/pingcap/tidb/pull/8149)
+* Fix the issue that the statistics of deleted tables are not cleared up for a long period of time [#8182](https://github.com/pingcap/tidb/pull/8182)
+### Server
+* Improve the readability of logs and make logs better
+    - [#8063](https://github.com/pingcap/tidb/pull/8063)
+    - [#8053](https://github.com/pingcap/tidb/pull/8053)
+    - [#8224](https://github.com/pingcap/tidb/pull/8224)
+* Fix the error occurred when obtaining the table data of `infoschema.profiling` [#8096](https://github.com/pingcap/tidb/pull/8096)
+* Replace the unix socket with the pumps client to write binlogs [#8098](https://github.com/pingcap/tidb/pull/8098)
+* Add the threshold value for the `tidb_slow_log_threshold` environment variable, which dynamically sets the slow log [#8094](https://github.com/pingcap/tidb/pull/8094)
+* Add the original length of a SQL statement truncated while the `tidb_query_log_max_len` environment variable dynamically sets logs [8200](https://github.com/pingcap/tidb/pull/8200)
+* Add the `tidb_opt_write_row_id` environment variable to control whether to allow writing `_tidb_rowid` [#8218](https://github.com/pingcap/tidb/pull/8218)
+* Add an upper bound to the `Scan` command of ticlient, to avoid overbound scan [#8081](https://github.com/pingcap/tidb/pull/8081), [#8247](https://github.com/pingcap/tidb/pull/8247)    
+### DDL
+* Fix the issue that executing DDL statements in transactions encounters an error in some cases [#8056](https://github.com/pingcap/tidb/pull/8056)
+* Fix the issue that executing `truncate table` in partition tables does not take effect [#8103](https://github.com/pingcap/tidb/pull/8103)
+* Fix the issue that the DDL operation does not roll back correctly after being cancelled in some cases [#8057](https://github.com/pingcap/tidb/pull/8057) 
+* Add the `admin show next_row_id` command to return the next available row ID [#8268](https://github.com/pingcap/tidb/pull/8268)  
+
+
 ## [2.1.0-rc.4] - 2018-10-23
 ### SQL Optimizer
 * Fix the issue that column pruning of `UnionAll` is incorrect in some cases [#7941](https://github.com/pingcap/tidb/pull/7941)