[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,

-----Original Message-----
From: RUBINO, Dana, GBM
Sent: 30 September 2008 15:59
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?


The Royal Bank of Scotland plc. Registered in Scotland No 90312. Registered Office: 36 St Andrew Square, Edinburgh EH2 2YB. 
Authorised and regulated by the Financial Services Authority 
This e-mail message is confidential and for use by the 
addressee only. If the message is received by anyone other 
than the addressee, please return the message to the sender 
by replying to it and then delete the message from your 
computer. Internet e-mails are not necessarily secure. The 
Royal Bank of Scotland plc does not accept responsibility for 
changes made to this message after it was sent. 

Whilst all reasonable care has been taken to avoid the 
transmission of viruses, it is the responsibility of the recipient to 
ensure that the onward transmission, opening or use of this 
message and any attachments will not adversely affect its 
systems or data. No responsibility is accepted by The 
Royal Bank of Scotland plc in this regard and the recipient should carry 
out such virus and other checks as it considers appropriate. 
Visit our websites at: 

More information about the erlang-questions mailing list