[erlang-questions] select error when using fragmented tables

[Dan Gudmundsson]

Thanks
I will take a look at it.

/Dan

Primanathan Reddy [ MTN - Innovation Centre ] wrote:
> Hello
> 
>  
> 
> I’m using mnesia:select/4 in an activity with the mnesia_frag access module.
> 
> Erlang R11B
> 
>  
> 
> I get the following error:
> 
>  
> 
> ** exited: {{badmatch,[{tab_frag2,node@REDACTED,disc_copies}]},
> 
>             [{mnesia_frag,init_select,6},
> 
>              {mnesia_tm,non_transaction,5},
> 
>              {erl_eval,do_apply,5},
> 
>              {shell,exprs,6},
> 
>              {shell,eval_loop,3}]} **
> 
>  
> 
>  
> 
> % --------------------
> 
> % Original Code
> 
> % ---------------------
> 
> init_select(Tid,Opaque,*Tab*,Pat,Limit,LockKind) ->
> 
>     case ?catch_val({Tab, frag_hash}) of
> 
>             {'EXIT', _} ->
> 
>                 mnesia:select(Tid, Opaque, Tab, Pat, Limit,LockKind);
> 
>             FH ->
> 
>                 FragNumbers = verify_numbers(FH,Pat),
> 
>                 Fun = fun(Num) ->
> 
>                                       Name = n_to_frag_name(Tab, Num),
> 
>                                       Node = val({Name, where_to_read}),
> 
>                                       Storage = 
> mnesia_lib:storage_type_at_node(Node, Name),
> 
>                                       mnesia:lock(Tid, Opaque, {table, 
> Name}, LockKind),
> 
>                                       {Name, Node, Storage}
> 
>                           end,
> 
>                 [{*Tab*,Node,Type}|NameNodes] = *lists:map*(Fun, 
> FragNumbers),
> 
>                 InitFun = fun(FixedSpec) -> 
> mnesia:dirty_sel_init(Node,Tab,FixedSpec,Limit,Type) end,
> 
>                 Res = 
> mnesia:fun_select(Tid,Opaque,Tab,Pat,LockKind,Tab,InitFun,Limit,Node,Type),
> 
>                 frag_sel_cont(Res, NameNodes, {Pat,LockKind,Limit})
> 
>     end.
> 
>  
> 
> After investigating I found that mnesia_frag:init_select/6 is trying to 
> match the result from  lists:map( Fun, FragNumbers )  to 
> [{*Tab*,Node,Type}|NameNodes], where Tab is the name of fragmented 
> table. This will work in all the cases where key is not specified as a 
> part of match specification. However, if the key was specified, instead 
> of Tab, the name of the fragment where key resides is returned instead, 
> which causes the code above to crash. The patch for it is provided 
> below, so I’d like to ask ERLANG/OTP team to include it, or provide 
> equivalent fix for it.
> 
>  
> 
>  
> 
>  
> 
> % ---------
> 
> % Patch
> 
> % ---------
> 
>  
> 
> init_select(Tid,Opaque,Tab,Pat,Limit,LockKind) ->
> 
>     case ?catch_val({Tab, frag_hash}) of
> 
>             {'EXIT', _} ->
> 
>                 mnesia:select(Tid, Opaque, Tab, Pat, Limit,LockKind);
> 
>             FH ->
> 
>                 FragNumbers = verify_numbers(FH,Pat),
> 
>                 Fun = fun(Num) ->
> 
>                                       Name = n_to_frag_name(Tab, Num),
> 
>                                       Node = val({Name, where_to_read}),
> 
>                                       Storage = 
> mnesia_lib:storage_type_at_node(Node, Name),
> 
>                                       mnesia:lock(Tid, Opaque, {table, 
> Name}, LockKind),
> 
>                                       {Name, Node, Storage}
> 
>                           end,
> 
>                 [{*PTab*,Node,Type}|NameNodes] = lists:map(Fun, 
> FragNumbers),
> 
>                 InitFun = fun(FixedSpec) -> 
> mnesia:dirty_sel_init(Node,*PTab*,FixedSpec,Limit,Type) end,
> 
>                 Res = 
> mnesia:fun_select(Tid,Opaque,*PTab*,Pat,LockKind,Tab,InitFun,Limit,Node,Type),
> 
>                 frag_sel_cont(Res, NameNodes, {Pat,LockKind,Limit})
> 
>     end.
> 
>