[erlang-questions] Re: Some tips on Mnesia

[Rapsey]

On Mon, May 24, 2010 at 8:49 PM, Igor Ribeiro Sucupira <igorrs@REDACTED>wrote:

> On Mon, May 24, 2010 at 1:45 AM, Rapsey <rapsey@REDACTED> wrote:
> > About that key value approach. My solution is a bit different. Let's say
> I
> > have a database of nodes and store a list of IP addresses to them. I save
> > them like so:
> > {node,{address,NodeName}}, {IP,Port}
>
> I assume you meant {node, {address,NodeName}, {IP,Port}}, so that the
> key would be {address, NodeName}. Is that correct?
>
> The problem I was addressing was "adding a field in a flexible
> manner". With your solution, you would, for example, add a "status"
> field by inserting new keys like {status, NodeName}, with values that
> could be "active", "down" or whatever. Is that the idea?
>
> I'm just not sure you can efficiently retrieve all the information
> about a node. Won't the query for this key {'_',NodeName} perform a
> full table scan?
>

No it will not actually. I asked this question a on the mailing list not too
long ago. As long as you don't use variable binding ('$1'), it will use the
index on the first field. {node,{address,NodeName}} is the entire key,
because I store more than just node information. This way I can use the same
table for different kinds of data.


> Even if that query was very smart (or you knew in advance what fields
> you need), it would still need to retrieve several keys, which could
> even be in different nodes (in the case of a fragmented table).
> Certainly slower than reading one, longer record (as is the case with
> a "key-value" table).
>
>
Yes your solution might be better for fragmented tables. I don't really use
mnesia in a distributed environment, so I used something that fits well with
a single node. This way of saving data is a bit nicer, since you don't have
to do a proplists:get_value after mnesia:read. But you can still query for
more than one type of data at the same time.



Sergej





> So, in short, compared to the "key-value" approach, your solution
> favors retrieving a single field (only slightly, if the records are
> smaller than a block/page), but turns it slower to retrieve all the
> fields of an entity.
>
> Best regards.
> Igor.
>
> > This way I can query for that exact information mnesia:(dirty_)read, or I
> > can get a list of all node addresses with mnesia:select -
> > {node,{address,'_'}}, or get a list of all node information with
> {node,'_'},
> > or get all info about a node {node,{'_',NodeName}}.
> >
> >
> > Sergej
> >
> > On Sun, May 23, 2010 at 10:46 PM, Igor Ribeiro Sucupira <
> igorrs@REDACTED>wrote:
> >
> >> Second and third posts:
> >> http://igorrs.blogspot.com/2010/05/mnesia-one-year-later-part-2.html
> >> http://igorrs.blogspot.com/2010/05/mnesia-one-year-later-part-3.html
> >>
> >> On Thu, May 20, 2010 at 12:53 PM, Igor Ribeiro Sucupira
> >> <igorrs@REDACTED> wrote:
> >> > For those people who are starting to work with Mnesia, I'm writing a
> >> > short sequence of blog posts that are intended to point out some
> >> > relevant facts that don't seem to be well-known and could hurt you in
> >> > the beginning.
> >> >
> >> > I'll try to focus on the information that you can't google.  ;-)  But
> >> > maybe in the end I'll also summarize the important tips that I learned
> >> > from other blogs.
> >> >
> >> > Here is the first post:
> >> > http://igorrs.blogspot.com/2010/05/mnesia-one-year-later.html
> >> >
> >> > Good luck.
> >> > Igor.
> >> >
> >> > --
> >> > "The secret of joy in work is contained in one word - excellence. To
> >> > know how to do something well is to enjoy it." - Pearl S. Buck.
> >>
> >> ________________________________________________________________
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> >>
> >>
> >
>
>
>
> --
> "The secret of joy in work is contained in one word - excellence. To
> know how to do something well is to enjoy it." - Pearl S. Buck.
>