Best way to print out Mnesia table

Question

I tried this code snippet:

print_next(Current) ->
    case mnesia:dirty_next(muppet, Current) of
        '$end_of_table' ->
            io:format("~n", []),
            ok;
        Next ->
            [Muppet] = mnesia:dirty_read({muppet, Next}),
            io:format("~p~n", [Muppet]),
            print_next(Next),
            ok
    end.

print() ->
    case mnesia:dirty_first(muppet) of
        '$end_of_table' ->
            ok;
        First ->
            [Muppet] = mnesia:dirty_read({muppet, First}),
            io:format("~p~n", [Muppet]),
            print_next(First),
            ok
    end.

But it is so long. Also I can use dirty_all_keys and then iterate through key list, but I want to know if there is a better way to print out Mnesia table contents.

Candelabra

If you are just looking for a way to browse a table, you can also use the table visualizer. – Tear

Answer 1

Well, if the intent is to see the contents of your table, there is the application called tv, which can view both ETS and mnesia tables.

If you wish to see all the table contents on your terminal, then try something like this:

traverse_table_and_show(Table_name)->
    Iterator =  fun(Rec,_)->
                    io:format("~p~n",[Rec]),
                    []
                end,
    case mnesia:is_transaction() of
        true -> mnesia:foldl(Iterator,[],Table_name);
        false -> 
            Exec = fun({Fun,Tab}) -> mnesia:foldl(Fun, [],Tab) end,
            mnesia:activity(transaction,Exec,[{Iterator,Table_name}],mnesia_frag)
    end.

Then if your table is called muppet, you use the function as follows:

traverse_table_and_show(muppet).

Advantages of this:
If its executed within a transaction , it will have no problems of nested transactions. It is less work because its done within one mnesia transaction through mnesia iterator functionality as compared to your implementation of get_next_key -> do_read_with_key -> then read the record (these are many operations). With this, mnesia will automatically tell that it has covered all the records in your entire table. Also, if the table is fragmented, your functionality will only display records in the first fragment. This will iterate through all the fragments the belong to that table.

In this iteration mnesia method, i do nothing with the Accumulator variable which should go along with the Iterator fun and thats why you see the underscore for the second variable.

Details of this iteration can be found here: http://www.erlang.org/doc/man/mnesia.html#foldl-3

Whinchat

Answer 2

If you just want a quick and dirty way to print the contents of a Mnesia table in the shell, and if your table is not of type disc_only_copies, then you can take advantage of the fact that Mnesia stores its data in ETS tables and run:

ets:tab2list(my_table).

or, if you think the shell truncates the data too much:

rp(ets:tab2list(my_table)).

Not recommended for "real" code, of course.

Ul

if data is too much, that list may hog the shell process. beware the data size in the ETS table before doing this – Whinchat

Answer 3

For a simple and quick look at your table contents you can use select function of mnesia with catch-all Match Specification as follows:

CatchAll = [{'_',[],['$_']}].
mnesia:dirty_select(TableName, CatchAll).

and also you can run it inside a transaction context:

CatchAll = [{'_',[],['$_']}].
SelectFun = fun() -> mnesia:select(TableName, CatchAll) end.
mnesia:transaction(SelectFun).

however be careful if you are in a production environment with a big data.

Blond

Answer 4

As Muzaaya told, you can you use tv (table visualizer tool) to view both mnesia and ets tables. Alternatively, you can use the following code to get mnesia table data - Print on terminal or in case you want to store the result in a file :

select_all() -> 
   mnesia:transaction( 
fun() ->
    P=qlc:e(qlc:q([E || E <- mnesia:table(tableName)])),      %query to select all data from table named 'tableName'                                                                                    
    io:format(" ~p ~n ", [P]), % Prints table data on terminal
    to_file("fileName.txt",P) % to_file method writes the data to file
end ).


to_file(File, L) ->
  mnesia:transaction( 
fun() ->
      {ok, S} = file:open(File, write),
      lists:foreach(fun(X) -> io:format(S, "~p.~n" ,[X]) end, L),
      file:close(S)
  end).

Aspirate