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    <p>Since Mnesia queries are just erlang functions I would imagine you can profile them the same way you would profile your own erlang code. <a href="http://www.erlang.org/doc/efficiency_guide/profiling.html#id2266192" rel="nofollow noreferrer">http://www.erlang.org/doc/efficiency_guide/profiling.html#id2266192</a> has more information on the erlang profiling tools available.</p> <p><strong>Update</strong> As a test I ran this at home on a test mnesia instance and using fprof to trace an mnesia qlc query returned output a sample of which I'm including below. So it definitely includes more information than just the query call.</p> <pre><code>.... {[{{erl_lint,pack_errors,1}, 2, 0.004, 0.004}], { {lists,map,2}, 2, 0.004, 0.004}, % [ ]}. {[{{mnesia_tm,arrange,3}, 1, 0.004, 0.004}], { {ets,first,1}, 1, 0.004, 0.004}, % [ ]}. {[{{erl_lint,check_remote_function,5}, 2, 0.004, 0.004}], { {erl_lint,check_qlc_hrl,5}, 2, 0.004, 0.004}, % [ ]}. {[{{mnesia_tm,multi_commit,4}, 1, 0.003, 0.003}], { {mnesia_locker,release_tid,1}, 1, 0.003, 0.003}, % [ ]}. {[{{mnesia,add_written_match,4}, 1, 0.003, 0.003}], { {mnesia,add_match,3}, 1, 0.003, 0.003}, % [ ]}. {[{{mnesia_tm,execute_transaction,5}, 1, 0.003, 0.003}], { {erlang,erase,1}, 1, 0.003, 0.003}, % [ ]}. {[{{mnesia_tm,intercept_friends,2}, 1, 0.002, 0.002}], { {mnesia_tm,intercept_best_friend,2}, 1, 0.002, 0.002}, % [ ]}. {[{{mnesia_tm,execute_transaction,5}, 1, 0.002, 0.002}], { {mnesia_tm,flush_downs,0}, 1, 0.002, 0.002}, % [ ]}. {[{{mnesia_locker,rlock_get_reply,4}, 1, 0.002, 0.002}], { {mnesia_locker,opt_lookup_in_client,3}, 1, 0.002, 0.002}, % [ ]}. {[ ], { undefined, 0, 0.000, 0.000}, % [{{shell,eval_exprs,6}, 0, 18.531, 0.000}, {{shell,exprs,6}, 0, 0.102, 0.024}, {{fprof,just_call,2}, 0, 0.034, 0.027}]}. </code></pre>
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    1. POProfiling Mnesia Queries
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    1. COAs Gordon already suggested, these tools would only yield something like "you're spending too much time in mnesia:match_object and qlc:eval". While I guess a real answer would be "you are joining against a non-indexed column here and there, making your mnesia slow".
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    2. COnot really. qlc:eval and mnesia:match_object call other erlang functions. I'm sure you could get more information than just your spending most of your time in mnesia:match_object. I'll see if I can try it out to be sure though.
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    3. COSo did this fprof trace give you any hints on how you should alter your schema?
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