[erlang-questions] Binary pattern matching on ETS

[Dmitry Kolesnikov]

Hello,

> I needed this prefix matching on gproc


You should mentioned that earlier. Use tuples as keys as Lukas said.

- Dmitry

> On 28 Oct 2014, at 03:20, Eduardo Gurgel <edgurgel@REDACTED> wrote:
> 
> Thank you, Lukas and Ulf. I'll check out both proposed solutions.
> 
> I needed this prefix matching on gproc, so I don't think kvdb will be a possible solution for me in this case .I will probably need to see if I can use a simple string instead of a binary to tag my processes.
> 
> Regards,
> 
> --
> 
> Eduardo
> 
> On 27 October 2014 23:06, Ulf Wiger <ulf@REDACTED <mailto:ulf@REDACTED>> wrote:
> 
> FWIW, the kvdb database [1] allows for select operations on e.g. ets-based tables, including prefix match on binary keys.
> 
> It does this through a little trickery. Since the match syntax doesn’t support prefix notation for binaries, and kvdb keys are always some form of binary encoding, specifying a list prefix, e.g. “aa” ++ ‘_’ in the head pattern will indicate to kvdb that you want to match on prefix, and the match spec will be modified. More specifically, kvdb will derive a proper prefix and do a prefix_match using ‘next’ iteration, and then matching the objects using ets:match_spec_run/2.
> 
> Example:
> 
> Eshell V5.10.3  (abort with ^G)
> 1> kvdb:start().
> starting kvdb
> KVDB Dbs = []
> ok
> 2> kvdb:open(db,[{backend,ets}]).
> File = "db.db"
> {ok,<0.53.0>}
> 3> kvdb:add_table(db,t,[]).
> ok
> 4> kvdb:put(db,t,{<<"aaaaa">>,1}).
> ok
> 5> kvdb:put(db,t,{<<"aabbb">>,2}).
> ok
> 6> kvdb:put(db,t,{<<"aabcc">>,3}).
> ok
> 7> kvdb:select(db,t,[{{<<"aa">>,'_'},[],['$_']}]).       
> {[],#Fun<kvdb_direct.34.51139377>}
> 8> kvdb:select(db,t,[{{"aa" ++ '_','_'},[],['$_']}]).
> {[{<<"aaaaa">>,1},{<<"aabbb">>,2},{<<"aabcc">>,3}],
>  #Fun<kvdb_direct.34.51139377>}
> 9> kvdb:select(db,t,[{{"aab" ++ '_','_'},[],['$_']}]).
> {[{<<"aabbb">>,2},{<<"aabcc">>,3}],
>  #Fun<kvdb_direct.34.51139377>}
> 
> This is obviously (significantly) slower than doing a normal ets:select() or mnesia:[dirty_]select(), but it’s mainly a question of constant-factor overheads. Kvdb clearly puts semantics before performance. ;-)
> 
> BR,
> Ulf W
> 
> [1] https://github.com/Feuerlabs/kvdb <https://github.com/Feuerlabs/kvdb>
> On 27 Oct 2014, at 10:02, Lukas Larsson <lukas@REDACTED <mailto:lukas@REDACTED>> wrote:
> 
>> Hello,
>> 
>> On Thu, Oct 23, 2014 at 11:35 AM, Eduardo Gurgel <edgurgel@REDACTED <mailto:edgurgel@REDACTED>>wrote:
>> Is it possible to match the prefix of a binary using ETS match spec?
>> 
>> It is not possible.
>>  
>> And if it's not, is it a limitation or just a missing implementation?
>> 
>> At the moment it is both :)
>> 
>> I've never tried to do something like that, but I would guess you work around this limitation by extracting what you have to match from the binary into a surrounding tuple before doing the insert. Right now there is afaik no plan to add the possibility to do this.
>> 
>> Lukas
>> _______________________________________________
>> erlang-questions mailing list
>> erlang-questions@REDACTED <mailto:erlang-questions@REDACTED>
>> http://erlang.org/mailman/listinfo/erlang-questions <http://erlang.org/mailman/listinfo/erlang-questions>
> 
> Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
> http://feuerlabs.com <http://feuerlabs.com/>
> 
> 
> 
> 
> 
> 
> -- 
> Eduardo
> _______________________________________________
> erlang-questions mailing list
> erlang-questions@REDACTED <mailto:erlang-questions@REDACTED>
> http://erlang.org/mailman/listinfo/erlang-questions <http://erlang.org/mailman/listinfo/erlang-questions>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20141028/6badbf2e/attachment.htm>