Using ets function to read mnesia table (erlang)

Question

working on an erlang project using mnesia (some tables ram copies, some tables disk copies, some tables both). in an attempt to optimize a certain read (ram table), i used the ets lookup rather than the mnesia dirty_read i had been using, and timed both versions of the routine. the ets lookup was significantly faster than the mnesia dirty_read.

my question is whether there is some 'gotcha' or 'catch' to reading an mnesia table using ets vs mnesia (there must be, otherwise there is no reason for the slower mnesia read to exist). if it makes any difference, i don't need and am not using any "distrubuted" or "nodes." in other words, i am and will only be using a single node on a single computer.

Answer 1

mnesia:dirty_read does a rpc call even if the table is local. Also it checks for the current activity context and maintains it even for dirty lookups. This will result in the extra time required for the lookup.

In your case (where there is only one node with local mnesia), direct ets lookup should work but not recommended as it will be implementation dependent. The better would be to use mnesia:ets(Fun,[, Args]).