• Dmitry Lenev's avatar
    Fix for bug #12652385 - "61493: REORDERING COLUMNS · edfd31a0
    Dmitry Lenev authored
    TO POSITION FIRST CAN CAUSE DATA TO BE CORRUPTED".
     
    ALTER TABLE MODIFY/CHANGE ... FIRST did nothing except renaming
    columns if new version of the table had exactly the same 
    structure as the old one (i.e. as result of such statement, names 
    of columns changed their order as specified but data in columns 
    didn't). The same thing happened for ALTER TABLE DROP COLUMN/ADD 
    COLUMN statements which were supposed to produce new version of
    table with exactly the same structure as the old version of table.
    I.e. in the latter case the result was the same as if old column 
    was renamed instead of being dropped and new column with default
    as value being created.
     
    Both these problems were caused by the fact that ALTER TABLE
    implementation incorrectly interpreted both these situations as 
    simple renaming of columns and assumed that in-place ALTER TABLE
    algorithm could have been used for them.
     
    This patch fixes this problem by ensuring that in cases when some
    column is moved to the first position or some column is dropped
    the default ALTER TABLE algorithm involving table copying is 
    always used. This is achieved by detecting such situations in
    mysql_prepare_alter_table() and setting Alter_info::change_level
    to ALTER_TABLE_DATA_CHANGED for them.
    
    mysql-test/r/alter_table.result:
      Added test for bug #12652385 - "61493: REORDERING COLUMNS TO
      POSITION FIRST CAN CAUSE DATA TO BE CORRUPTED".
    mysql-test/t/alter_table.test:
      Added test for bug #12652385 - "61493: REORDERING COLUMNS TO
      POSITION FIRST CAN CAUSE DATA TO BE CORRUPTED".
    sql/sql_table.cc:
      Changed mysql_prepare_alter_table() to detect situations in 
      which we some column moved to the first position or some column 
      is dropped and ensure that such ALTER TABLE statements won't
      be carried out using in-place algorithm. The latter could have 
      happened before this patch if new version of table had the same 
      structure as the old one (except the column names).
    edfd31a0
alter_table.result 45.5 KB