What is mnesia:select/2 used for?

mnesia:select/2 queries a table using a match specification, letting you filter on conditions beyond a simple primary-key lookup — the same match-spec mechanism ETS uses, run inside a Mnesia transaction for consistency.

mnesia:transaction(fun() ->
    mnesia:select(person,
        [{#person{age = '$1', _ = '_'}, [{'>', '$1', 30}], ['$_']}])
end).
%% returns every person record with age > 30

Where mnesia:read/1 only works by primary key, select/2 can filter on any field, including combinations of conditions, at the cost of scanning more of the table (unless an index exists on the filtered field). Like other match-spec code, it's often written using ets:fun2ms/1-style helpers to avoid hand-crafting the raw match specification tuple.

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