[erlang-questions] qlc cursor offset

Alain O'Dea <>
Tue Dec 28 17:18:55 CET 2010


I code-shopped Paul Mineiro's solution when I needed to do efficient
pagination with Mnesia like this for scrumjet:

% Author: Paul Mineiro
% SOURCE: http://erlanganswers.com/web/mcedemo/mnesia/OrderedBy.html
-module (scrumjet_mnesia).
-export ([ limit/3 ]).

limit (Tab, Offset, Number) ->
  seek (Offset,
        Number,
        mnesia:select (Tab,
                       [ { mnesia:table_info (Tab, wild_pattern), [],
[ '$_' ] } ],
                       10,
                       read)).

seek (_Offset, _Number, '$end_of_table') ->
  [];
seek (Offset, Number, X) when Offset =< 0 ->
  read (Number, X, []);
seek (Offset, Number, { Results, Cont }) ->
  NumResults = length (Results),
  case Offset > NumResults of
    true ->
      seek (Offset - NumResults, Number, mnesia:select (Cont));
    false ->
      { _, DontDrop } = lists:split (Offset, Results),
      Keep = lists:sublist (DontDrop, Number),
      read (Number - length (Keep), mnesia:select (Cont), [ Keep ])
  end.

read (Number, _, Acc) when Number =< 0 ->
  lists:foldl (fun erlang:'++'/2, [], Acc);
read (_Number, '$end_of_table', Acc) ->
  lists:foldl (fun erlang:'++'/2, [], Acc);
read (Number, { Results, Cont }, Acc) ->
  NumResults = length (Results),
  case Number > NumResults of
    true ->
      read (Number - NumResults, mnesia:select (Cont), [ Results | Acc ]);
    false ->
      { Keep, _ } = lists:split (Number, Results),
      lists:foldl (fun erlang:'++'/2, Keep, Acc)
  end.

The source of the scrumjet_mnesia can be downloaded conveniently from:
https://github.com/AlainODea/scrumjet/blob/master/src/scrumjet_mnesia.erl

Paul Mineiro's original solution is here:
http://erlanganswers.com/web/mcedemo/mnesia/OrderedBy.html

On Tue, Dec 28, 2010 at 12:05 PM, Andre Nathan <>wrote:

> Hello
>
> Is there a way to move the qlc cursor to a given offset without fetching
> all the intermediate results? The idea is to implement pagination and
> allow a user to jump to a specific page without the need to retrieve all
> the data from previous pages.
>
> My mnesia table is something like this:
>
> -record(quota, {
>  user,
>  domain_id,
>  count_cur,
>  count_max
> }).
>
> and I'd like to fetch the quotas for all users of a given domain_id, a
> few each time.
>
> Thanks in advance
> Andre
>
>
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