docs: update change-log add 2.0.6 (#7285)

Author: coocood

CHANGELOG.md

@@ -2,6 +2,37 @@
 
 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/pingcap/tikv/blob/master/CHANGELOG.md) and [PD changelog](https://github.com/pingcap/pd/blob/master/CHANGELOG.md).
 
+## [2.0.6] - 2018-08-06
+### Improvements
+  - Make “set system variable” log shorter to save disk space [#7031](https://github.com/pingcap/tidb/pull/7031)
+  - Record slow operations during the execution of `ADD INDEX` in the log, to make troubleshooting easier [#7083](https://github.com/pingcap/tidb/pull/7083)
+  - Reduce transaction conflicts when updating statistics [#7138](https://github.com/pingcap/tidb/pull/7138)
+  - Improve the accuracy of row count estimation when the values pending to be estimated exceeds the statistics range [#7185](https://github.com/pingcap/tidb/pull/7185)
+  - Choose the table with a smaller estimated row count as the outer table for `Index Join` to improve its execution efficiency [#7277](https://github.com/pingcap/tidb/pull/7277)
+  - Add the recover mechanism for panics occurred during the execution of `ANALYZE TABLE`, to avoid that the tidb-server is unavailable caused by abnormal behavior in the process of collecting statistics [#7228](https://github.com/pingcap/tidb/pull/7228)
+  - Return `NULL` and the corresponding warning when the results of `RPAD`/`LPAD` exceed the value of the `max_allowed_packet` system variable, compatible with MySQL [#7244](https://github.com/pingcap/tidb/pull/7244)
+  - Set the upper limit of placeholders count in the `PREPARE` statement to 65535, compatible with MySQL [#7250](https://github.com/pingcap/tidb/pull/7250)
+### Bug Fixes
+  - Fix the issue that the `DROP USER` statement is incompatible with MySQL behavior in some cases [#7014](https://github.com/pingcap/tidb/pull/7014)
+  - Fix the issue that statements like `INSERT`/`LOAD DATA` meet OOM aftering opening `tidb_batch_insert` [#7092](https://github.com/pingcap/tidb/pull/7092)
+  - Fix the issue that the statistics fail to automatically update when the data of a table keeps updating [#7093](https://github.com/pingcap/tidb/pull/7093)
+  - Fix the issue that the firewall breaks inactive gPRC connections [#7099](https://github.com/pingcap/tidb/pull/7099)
+  - Fix the issue that prefix index returns a wrong result in some scenarios [#7126](https://github.com/pingcap/tidb/pull/7126)
+  - Fix the panic issue caused by outdated statistics in some scenarios [#7155](https://github.com/pingcap/tidb/pull/7155)
+  - Fix the issue that one piece of index data is missed after the `ADD INDEX` operation in some scenarios [#7156](https://github.com/pingcap/tidb/pull/7156)
+  - Fix the wrong result issue when querying `NULL` values using the unique index in some scenarios [#7172](https://github.com/pingcap/tidb/pull/7172)
+  - Fix the messy code issue of the `DECIMAL` multiplication result in some scenarios [#7212](https://github.com/pingcap/tidb/pull/7212)
+  - Fix the wrong result issue of `DECIMAL` modulo operation in some scenarios [#7245](https://github.com/pingcap/tidb/pull/7245)
+  - Fix the issue that the `UPDATE`/`DELETE` statement in a transaction returns a wrong result under some special sequence of statements [#7219](https://github.com/pingcap/tidb/pull/7219)
+  - Fix the panic issue of the `UNION ALL`/`UPDATE` statement during the process of building the execution plan in some scenarios [#7225](https://github.com/pingcap/tidb/pull/7225)
+  - Fix the issue that the range of prefix index is calculated incorrectly in some scenarios [#7231](https://github.com/pingcap/tidb/pull/7231)
+  - Fix the issue that the `LOAD DATA` statement fails to write the binlog in some scenarios [#7242](https://github.com/pingcap/tidb/pull/7242)
+  - Fix the wrong result issue of `SHOW CREATE TABLE` during the execution process of `ADD INDEX` in some scenarios [#7243](https://github.com/pingcap/tidb/pull/7243)
+  - Fix the issue that panic occurs when `Index Join` does not initialize timestamps in some scenarios [#7246](https://github.com/pingcap/tidb/pull/7246)
+  - Fix the false alarm issue when `ADMIN CHECK TABLE` mistakenly uses the timezone in the session [#7258](https://github.com/pingcap/tidb/pull/7258)
+  - Fix the issue that `ADMIN CLEANUP INDEX` does not clean up the index in some scenarios [#7265](https://github.com/pingcap/tidb/pull/7265)
+  - Disable the Read Committed isolation level [#7282](https://github.com/pingcap/tidb/pull/7282)
+
 ## [2.0.5] - 2018-07-06
 ### New Features
   - Add the `tidb_disable_txn_auto_retry` system variable which is used to disable the automatic retry of transactions [#6877](https://github.com/pingcap/tidb/pull/6877)