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

[Robert Virding]

As I read it in this case Mnesia cannot use the index as the key contains a variable and Mnesia hashes the key for indexed lookup. So in this case Mnesia must do a scan. This is the same as for ETS. 

Robert 

----- Original Message -----

> From: "Evgeny M" <donpedrothird@REDACTED>
> To: erlang-programming@REDACTED
> Cc: erlang-questions@REDACTED
> Sent: Monday, 27 May, 2013 12:33:02 AM
> Subject: Re: [erlang-questions] Mnesia - select with placeholder in
> table key

> 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 >:
> 

> > > 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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