add changelog for 2.1.0 rc4 (#8020) (#8027)

Author: zz-jason

CHANGELOG.md

@@ -1,6 +1,37 @@
 # 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.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)
+* Fix the issue that the result of the `UnionAll` operator is incorrect in some cases [#8007](https://github.com/pingcap/tidb/pull/8007)
+### SQL Execution Engine
+* Fix the precision issue of the `AVG` function [#7874](https://github.com/pingcap/tidb/pull/7874)
+* Support using the `EXPLAIN ANALYZE` statement to check the runtime statistics including the execution time and the number of returned rows of each operator during the query execution process [#7925](https://github.com/pingcap/tidb/pull/7925)
+* Fix the panic issue of the `PointGet` operator when a column of a table appears multiple times in the result set [#7943](https://github.com/pingcap/tidb/pull/7943)
+* Fix the panic issue caused by too large values in the `Limit` subclause [#8002](https://github.com/pingcap/tidb/pull/8002)
+* Fix the panic issue during the execution process of the `AddDate`/`SubDate` statement in some cases [#8009](https://github.com/pingcap/tidb/pull/8009)
+### Statistics
+* Fix the issue of judging the prefix of the histogram low-bound of the combined index as out of range [#7856](https://github.com/pingcap/tidb/pull/7856)
+* Fix the memory leak issue caused by statistics collecting [#7873](https://github.com/pingcap/tidb/pull/7873)
+* Fix the panic issue when the histogram is empty [#7928](https://github.com/pingcap/tidb/pull/7928)
+* Fix the issue that the histogram bound is out of range when the statistics is being uploaded [#7944](https://github.com/pingcap/tidb/pull/7944)
+* Limit the maximum length of values in the statistics sampling process [#7982](https://github.com/pingcap/tidb/pull/7982)
+### Server
+* Refactor Latch to avoid misjudgment of transaction conflicts and improve the execution performance of concurrent transactions [#7711](https://github.com/pingcap/tidb/pull/7711)
+* Fix the panic issue caused by collecting slow queries in some cases [#7874](https://github.com/pingcap/tidb/pull/7847)
+* Fix the panic issue when `ESCAPED BY` is an empty string in the `LOAD DATA` statement [#8005](https://github.com/pingcap/tidb/pull/8005)
+* Complete the “coprocessor error” log information [#8006](https://github.com/pingcap/tidb/pull/8006)
+### Compatibility
+* Set the `Command` field of the `SHOW PROCESSLIST` result to `Sleep` when the query is empty [#7839](https://github.com/pingcap/tidb/pull/7839)
+### Expressions
+* Fix the constant folding issue of the `SYSDATE` function [#7895](https://github.com/pingcap/tidb/pull/7895)
+* Fix the issue that `SUBSTRING_INDEX` panics in some cases [#7897](https://github.com/pingcap/tidb/pull/7897)
+### DDL
+* Fix the stack overflow issue caused by throwing the `invalid ddl job type` error [#7958](https://github.com/pingcap/tidb/pull/7958)
+* Fix the issue that the result of `ADMIN CHECK TABLE` is incorrect in some cases [#7975](https://github.com/pingcap/tidb/pull/7975)
+
+
 ## [2.1.0-rc.2] - 2018-09-14
 ### SQL Optimizer
 * Put forward a proposal of the next generation Planner [#7543](https://github.com/pingcap/tidb/pull/7543)
@@ -15,7 +46,7 @@ All notable changes to this project will be documented in this file. See also [R
 * Optimize the performance of Hash aggregate operators [#7541](https://github.com/pingcap/tidb/pull/7541)
 * Optimize the performance of Join operators [#7493](https://github.com/pingcap/tidb/pull/7493), [#7433](https://github.com/pingcap/tidb/pull/7433)
 * Fix the issue that the result of `UPDATE JOIN` is incorrect when the Join order is changed [#7571](https://github.com/pingcap/tidb/pull/7571)
-* Improve the performance of Chunk’s iterator [#7585](https://github.com/pingcap/tidb/pull/7585) 
+* Improve the performance of Chunk’s iterator [#7585](https://github.com/pingcap/tidb/pull/7585)
 ### Statistics
 * Fix the issue that the auto Analyze work repeatedly analyzes the statistics [#7550](https://github.com/pingcap/tidb/pull/7550)
 * Fix the statistics update error that occurs when there is no statistics change [#7530](https://github.com/pingcap/tidb/pull/7530)