DXF: add a limitation that adding indexes to columns of TIMESTAMP d…

…ata type via the DXF is not supported (#15787) (#15789)

mysql-schema.md

@@ -90,7 +90,6 @@ Currently, the `help_topic` is NULL.
 - `GLOBAL_VARIABLES`: global system variable table
 - `expr_pushdown_blacklist`: the blocklist for expression pushdown
 - `opt_rule_blacklist`: the blocklist for logical optimization rules
-- `table_cache_meta`: the metadata of cached tables
 - `tidb_import_jobs`: the job information of [`IMPORT INTO`](/sql-statements/sql-statement-import-into.md)
 - `tidb_timers`: the metadata of internal timers
 

tidb-distributed-execution-framework.md

@@ -60,6 +60,11 @@ Currently, for TiDB Self-Hosted, the TiDB backend task distributed execution fra
 
 - `IMPORT INTO` is used to import data in formats such as `CSV`, `SQL`, and `PARQUET` into an empty table. For more information, see [`IMPORT INTO`](https://docs.pingcap.com/tidb/v7.2/sql-statement-import-into).
 
+## Limitation
+
+- The DXF can only schedule the distributed execution of one `ADD INDEX` task at a time. If a new `ADD INDEX` task is submitted before the current `ADD INDEX` distributed task has finished, the new task is executed through a transaction.
+- Adding indexes on columns with the `TIMESTAMP` data type through the DXF is not supported, because it might lead to inconsistency between the index and the data.
+
 ## Prerequisites
 
 Before using the distributed framework, you need to enable the [Fast Online DDL](/system-variables.md#tidb_ddl_enable_fast_reorg-new-in-v630) mode.