[erlang-questions] Mnesia multi-table joins

Dana.RUBINO <>
Tue Sep 30 17:49:31 CEST 2008


Hi all,

Changed subject of my last thread as it wasn't a performance issue!

I have done some more digging and found that Mnesia appears to support only joins across two tables.

I have tried a cut down version of the below query using the two big tables only and it is lightning quick.

Does anyone know if there is a way to perform joins across more than two tables?

I fear I will have tripped on the last hurdle (I am currently prototyping a proposed system) if I cant perform joins across more than two tables.

Many Thanks,
Dan

-----Original Message-----
From: RUBINO, Dana, GBM
Sent: 30 September 2008 15:59
To: 
Subject: Extremely poor Mnesia performance


Hey all,

I am doing some benchmarking with Mnesia at the moment and am pretty disappointed to say the least.

I'm hoping I am doing something wrong.

Running a 5 table join below: (two of the tables have ~50k rows the rest a couple of hundred)

execute(qlc:q([I#i.user_id       ||   I <- mnesia:table(i),
                                                C <- mnesia:table(c),
                                                I#i.user_id =:= C#c.user_id,
                                                I#i.group =:= "Group A",
                                                P <- mnesia:table(p),
                                                T <- mnesia:table(t),
                                                S <- mnesia:table(s),
                                                C#c.platform_id =:= P#p.platform_id,
                                                C#c.trans_id =:= T#t.trans_id,
                                                C#c.sales_id =:= S#s.sales_id
                                                ])).

On a normal RDBMS this query returns in < 1min. I am yet to get a result from Mnesia and its been running for 10 mins.

Is there anyway I can see what's going wrong with this query under the hood?

Thanks,
Dan

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