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POProfiling Mnesia Queries
 

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    <p>Our Mnesia DB is running slowly and we think it should be somewhat faster. </p> <p>So we need to profile it and work out what is happening.</p> <p>There are a number of options that suggest themselves:</p> <ul> <li>run fprof and see where the time is going</li> <li>run cprof and see which functions are called a lot</li> </ul> <p>However these are both fairly standard performance monitoring style tools. The question is how do I actually do query profiling - which queries are taking the longest times. If we were an Oracle or MySQL shop we would just run a query profiler which would return the sorts of queries that were taking a long time to run. This is not a tool that appears to be available for Mnesia.</p> <p>So the question is:</p> <ul> <li>what techniques exist to profile Mnesia</li> <li>what tools exist to profile Mnesia - none I think, but prove me wrong :)</li> <li>how did you profile your queries and optimise your mnesia database installation</li> </ul> <h2>Expanded In The Light Of Discussion</h2> <p>One of the problems with fprof as a profiling tool is that it only tells you about the particular query you are looking at. So fprof tells me that X is slow and I tweak it down to speed it up. Then, low and behold, operation Y (which was fast enough) is now dog slow. So I profile up Y and realise that the way to make Y quick is to make X slow. So I end up doing a series of bilateral trade-offs...</p> <p>What I actually <strong>need</strong> is a way to manage multilateral trade-offs. I now have 2 metric shed-loads of actual user activities logged which I can replay. These logs represent what I would like to optimize.</p> <p>A 'proper' query analyser on an SQL database would be able to profile the structure of SQL statements, eg all statements with the form: </p> <pre><code>SELECT [fieldset] FROM [table] WHERE {field = *parameter*}, {field = *parameter*} </code></pre> <p>and say <em>285 queries of this form took on average 0.37ms to run</em></p> <p>They magic answers are when it says: <em>17 queries of this form took 6.34s to run and did a full table scan on table X, you should put an index on field Y</em></p> <p>When I have a result set like this over a representative set of user-activities I can then start to reason about trade-offs in the round - and design a test pattern.</p> <p>The test pattern would be something like:</p> <ul> <li>activity X would make queries A, C and C faster but queries E and F slower</li> <li>test and measure</li> <li>then approve/disapprove</li> </ul> <p>I have been using Erlang long enough to 'know' that there is no query analyser like this, what I would like to know is how other people (who must have had this problem) 'reason' about mnesia optimization.</p>
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    1. USGordon Guthrie
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      1. USHassan Syed
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