Automerge.

include/myisammrg.h

@@ -88,7 +88,8 @@ extern MYRG_INFO *myrg_parent_open(const char *parent_name,
                                    void *callback_param);
 extern int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
                                 MI_INFO *(*callback)(void*),
-                                void *callback_param);
+                                void *callback_param,
+                                my_bool *need_compat_check);
 extern int myrg_detach_children(MYRG_INFO *m_info);
 extern int myrg_panic(enum ha_panic_function function);
 extern int myrg_rfirst(MYRG_INFO *file,uchar *buf,int inx);

mysql-test/r/merge.result

@@ -2127,4 +2127,18 @@ SELECT * FROM m1;
 ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
 DROP VIEW v1;
 DROP TABLE m1, t1;
+#
+# Bug #45796: invalid memory reads and writes when altering merge and 
+#             base tables
+#
+CREATE TABLE t1(c1 INT) ENGINE=MyISAM;
+CREATE TABLE m1(c1 INT) ENGINE=MERGE UNION=(t1);
+ALTER TABLE m1 ADD INDEX idx_c1(c1);
+SELECT * FROM m1;
+ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
+ALTER TABLE t1 ADD INDEX idx_c1(c1);
+SELECT * FROM m1;
+c1
+DROP TABLE m1;
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/merge.test

@@ -1535,4 +1535,24 @@ SELECT * FROM m1;
 
 DROP VIEW v1;
 DROP TABLE m1, t1;
+
+--echo #
+--echo # Bug #45796: invalid memory reads and writes when altering merge and 
+--echo #             base tables
+--echo #
+
+CREATE TABLE t1(c1 INT) ENGINE=MyISAM;
+CREATE TABLE m1(c1 INT) ENGINE=MERGE UNION=(t1);
+ALTER TABLE m1 ADD INDEX idx_c1(c1);
+# Open the MERGE table and allocate buffers based on children's definition.
+--error ER_WRONG_MRG_TABLE
+SELECT * FROM m1;
+# Change the child table definition.
+ALTER TABLE t1 ADD INDEX idx_c1(c1);
+# Check that old buffers are not reused
+SELECT * FROM m1;
+
+DROP TABLE m1;
+DROP TABLE t1;
+
 --echo End of 5.1 tests

storage/myisammrg/ha_myisammrg.cc

@@ -545,7 +545,8 @@ int ha_myisammrg::attach_children(void)
 
   if (myrg_attach_children(this->file, this->test_if_locked |
                            current_thd->open_options,
-                           myisammrg_attach_children_callback, this))
+                           myisammrg_attach_children_callback, this,
+                           (my_bool *) &need_compat_check))
   {
     DBUG_PRINT("error", ("my_errno %d", my_errno));
     DBUG_RETURN(my_errno ? my_errno : -1);

storage/myisammrg/myrg_open.c

@@ -365,11 +365,14 @@ MYRG_INFO *myrg_parent_open(const char *parent_name,
     The callback returns the MyISAM table handle of the child table.
     Check table definition match.
 
-  @param[in]    m_info          MERGE parent table structure
-  @param[in]    handle_locking  if contains HA_OPEN_FOR_REPAIR, warn about
-                                incompatible child tables, but continue
-  @param[in]    callback        function to call for each child table
-  @param[in]    callback_param  data pointer to give to the callback
+  @param[in]    m_info            MERGE parent table structure
+  @param[in]    handle_locking    if contains HA_OPEN_FOR_REPAIR, warn about
+                                  incompatible child tables, but continue
+  @param[in]    callback          function to call for each child table
+  @param[in]    callback_param    data pointer to give to the callback
+  @param[in]    need_compat_check pointer to ha_myisammrg::need_compat_check
+                                  (we need this one to decide if previously
+                                  allocated buffers can be reused).
 
   @return status
     @retval     0               OK
@@ -382,7 +385,7 @@ MYRG_INFO *myrg_parent_open(const char *parent_name,
 
 int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
                          MI_INFO *(*callback)(void*),
-                         void *callback_param)
+                         void *callback_param, my_bool *need_compat_check)
 {
   ulonglong  file_offset;
   MI_INFO    *myisam;
@@ -423,6 +426,11 @@ int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
       m_info->reclength= myisam->s->base.reclength;
       min_keys=  myisam->s->base.keys;
       key_parts= myisam->s->base.key_parts;
+      if (*need_compat_check && m_info->rec_per_key_part)
+      {
+        my_free((char *) m_info->rec_per_key_part, MYF(0));
+        m_info->rec_per_key_part= NULL;
+      }
       if (!m_info->rec_per_key_part)
       {
         if(!(m_info->rec_per_key_part= (ulong*)