Union ETS tables

Question

I try union of the two ETS tables into a single ETS table . The only way I know is to create a third table and insert the records of the two tables in a third table. Is there a better way?

Answer 1

ets:insert allows a list of tuples. On the other hand, ets:tab2list exports an ets table as a list of tuples. This means you can easily import one ets table into another one.

Not exactly a union, but you end up with a single table containing both previous ones, without creating a third ets table, which seems to be what you try to achieve.

A little example:

ets:new(list_a,[named_table]).
ets:new(list_b,[named_table]).

ets:insert(list_a,{one,1}).
ets:insert(list_b,{two,2}).
ets:insert(list_b,{three,3}).

ets:insert(list_a,ets:tab2list(list_b)).

ets:tab2list(list_a).
% list_a = [{three,3},{two,2},{one,1}]

What happens in case of identical keys depends on the type of ETS table you're working with (erlang doc):

If the table is a set and the key of the inserted objects matches the key of any object in the table, the old object will be replaced. If the table is an ordered_set and the key of the inserted object compares equal to the key of any object in the table, the old object is also replaced. If the list contains more than one object with matching keys and the table is a set, one will be inserted, which one is not defined. The same thing holds for ordered_set, but will also happen if the keys compare equal.

Besides, you might want to use ets:insert_new, which does not overwrite.