mnesia:delect_object只删除索引问题

前言

最近遇到一个奇怪的问题，mnesia数据有时能查出来有时查不出来，细查发现是mnesia:delect_object删除时，只删除了索引，没有删除具体的数据

问题重现

1. 表结构:

   -record(gordon_test, {

   idr, % {id, pid}
   id,
   pid,
   locale,
   time = os:timestamp()

   }).
   

2. 初使化代码:

   mnesia:create_table(gordon_test, [

   {ram_copies, [node() | nodes()]},
   {attributes, record_info(fields, gordon_test)}

   ]),
   mnesia:add_table_index(gordon_test, id),
   mnesia:add_table_index(gordon_test, pid).
   

3. 插入数据:

   mnesia:dirty_write(gordon_test, {gordon_test, {1, 1}, 1, 1, "", "", os:timestamp()}).
   mnesia:dirty_write(gordon_test, {gordon_test, {2, 2}, 2, 2, "", "", os:timestamp()}).
   mnesia:dirty_write(gordon_test, {gordon_test, {3, 3}, 3, 3, "", "", os:timestamp()}).
   

3.查询:

Id = 3,
Pid = 3,
% 查出id=3的所有记录，都能查出一条记录
mnesia:dirty_index_read(gordon_test, Id, #gordon_test.id),
mnesia:dirty_index_match_object(#gordon_test{idr = '$1',id=Id, _='_'}, #gordon_test.id),

1. 执行删除:

   mnesia:dirty_delete_object(X#gordon_test{locale = aaaa}).
   or
   Trans = fun() ->
       mnesia:delete_object(X#gordon_test{locale = aaaa})
           end,
   mnesia:transaction(Trans)
   

2. 两次查询，dirty_index_read查不到数据:

   % 数据查询为空
   mnesia:dirty_index_read(gordon_test, Id, #gordon_test.id),
   % 数据不变
   mnesia:dirty_index_match_object(#gordon_test{idr = '$1',id=Id, _='_'}, #gordon_test.id),
   

问题定位

查文档:

If a table is of type bag, it can sometimes be needed to delete only some of the records with a certain key. This can be done with the function delete_object/3. A complete record must be supplied to this function.

The semantics of this function is context-sensitive. For details, see mnesia:activity/4. In transaction-context, it acquires a lock of type LockKind on the record. Currently, the lock types write and sticky_write are supported.

深层研究

待继续 @todo