MDEV-28545 MyISAM reorganize partition corrupt older table format

The ALTER related code cannot do at the same time both:
- modify partitions
- change column data types

Explicit changing of a column data type together with a partition change is
prohibited by the parter, so this is not allowed and returns a syntax error:

  ALTER TABLE t MODIFY ts BIGINT, DROP PARTITION p1;

This fix additionally disables implicit data type upgrade
(e.g. from "MariaDB 5.3 TIME" to "MySQL 5.6 TIME", or the other way
around according to the current mysql56_temporal_format) in case of
an ALTER modifying partitions, e.g.:

  ALTER TABLE t DROP PARTITION p1;

In such commands now only the partition change happens, while
the data types stay unchanged.

One can additionally run:

  ALTER TABLE t FORCE;

either before or after the ALTER modifying partitions to
upgrade data types according to mysql56_temporal_format.

1 files changed, 12 insertions, 0 deletions

diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 6aae927800e..efa5c06dd2a 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -10068,6 +10068,18 @@ do_continue:;
 
   set_table_default_charset(thd, create_info, alter_ctx.db);
 
+  /*
+    The ALTER related code cannot alter partitions and change column data types
+    at the same time. So in case of partition change statements like:
+      ALTER TABLE t1 DROP PARTITION p1;
+    we skip implicit data type upgrade (such as "MariaDB 5.3 TIME" to
+    "MySQL 5.6 TIME" or vice versa according to mysql56_temporal_format).
+    Note, one can run a separate "ALTER TABLE t1 FORCE;" statement
+    before or after the partition change ALTER statement to upgrade data types.
+  */
+  if (IF_PARTITIONING(!fast_alter_partition, 1))
+    Create_field::upgrade_data_types(alter_info->create_list);
+
   if (create_info->check_fields(thd, alter_info,
                                 table_list->table_name, table_list->db) ||
       create_info->fix_period_fields(thd, alter_info))