[erlang-questions] Mnesia - select with placeholder in table key

[Evgeny M]

Sorry, I don't quite understand you. I'm not a native speaker, maybe I miss 
something.

We select by primary key only. There are no secondary indexes in the table, 
and there are no filters by other fields in the query. But the primary key 
is a tuple and one of the elements of this tuple is wildcard, ie we have 
records with keys {a, x, y}, {b, x, y}, {c, x, y}, {a, z, z} and want to 
select all records that match {'$1', x, y}. This tuple is the key, there 
aresome other fields in the record.
Will the mnesia use index in this case?

понедельник, 27 мая 2013 г., 1:17:38 UTC+4 пользователь Ulf Wiger написал:
>
> If the key position is unbound, mnesia will check if any indexed position 
> is bound, and if so, use the first such value for an index lookup. It then 
> uses match_spec_run() on the result. 
>
> BR, 
> Ulf W 
>
> Ulf Wiger, Feuerlabs, Inc. 
> http://www.feuerlabs.com 
>
> 26 maj 2013 kl. 22:46 skrev John Doe <donped...@REDACTED <javascript:>>: 
>
> > If I use select when the key is tuple with placeholders, something like 
> this (taken from erlang docs and changed a bit): 
> > MatchHead = #person{name = {'$1', "K", "Jameson"}, sex = '$2', _ = '_'}, 
> > Result = '$2', 
> > mnesia:select(Tab,[{MatchHead, [], [Result]}]) 
> >  where name is the table key 
> > will mnesia do full table scan in this case? Or it will be smarter and 
> select all records with key matching {_, "K", "Jameson"} using index 
> somehow? 
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