Added LINEAR HASH/KEY mysql compatibility

mysql-compatibility.md

@@ -150,6 +150,7 @@ In TiDB, all supported DDL changes are performed online. Compared with DDL opera
 * Table Partitioning supports `HASH`, `RANGE`, and `LIST` partitioning types. For the unsupported partition type, the `Warning: Unsupported partition type %s, treat as normal table` error might be output, where `%s` is a specific partition type.
 * Table Partitioning also supports `ADD`, `DROP`, and `TRUNCATE` operations. Other partition operations are ignored. The following Table Partition syntaxes are not supported:
     - `PARTITION BY KEY`
+    - `PARTITION BY LINEAR KEY`
     - `SUBPARTITION`
     - `{CHECK|TRUNCATE|OPTIMIZE|REPAIR|IMPORT|DISCARD|REBUILD|REORGANIZE|COALESCE} PARTITION`
 

partitioned-table.md

@@ -560,6 +560,15 @@ MOD(YEAR('2005-09-01'),4)
 =  1
 ```
 
+#### LINEAR HASH handling
+
+Before v6.4, if you execute DDL statements of MySQL Linear Hash partitions in TiDB, TiDB can only create non-partitioned tables. In this case, if you still want to use partitioned tables in TiDB, you need to update your DDL statements.
+
+Since v6.4.0, TiDB supports parsing the MySQL `PARTITION BY LINEAR HASH` syntax but ignores the `LINEAR` keyword in it. If you have some existing DDL and DML statements of MySQL Linear Hash partitions, you can execute them in TiDB without modification:
+
+- For DDL statements of MySQL Linear Hash partitions, TiDB will create non-linear Hash partitioned tables correspondingly (note that Linear Hash partitioned tables are not provided in TiDB). 
+- DML statements of MySQL Linear Hash partitions are not affected unless you use explicit partition selection, in which case TiDB will return the query results of the corresponding TiDB Hash partitioned normally.
+
 ### How TiDB partitioning handles NULL
 
 It is allowed in TiDB to use `NULL` as the calculation result of a partitioning expression.