[Tracker] The quadruple, team conversations on IRC

This is a mail describing what the team has been discussing on IRC, so
that people who aren't on IRC can follow up too.

o. We want to start having a quadruple alongside the decomposed sqlite

   o. Such a quadruple will allow us to implement backup

   o. Such a quadruple will allow us to implement named graph support

      o. Makes it possible for synchronization software to make somewhat
         intelligent decisions based on for example origin of the triple
      o. Allows us to store the origin of metadata (local, flickr,
         facebook, etc)
      o. Would make the KDE people interested in using Tracker for their
         mobile targets (according to discussions with them at the
         desktop summit conference earlier this month)
      o. Avoids the embedded/non-embedded metadata issue when updating
         indexed files as we'd know the origin for each statement

o. The quadruple will store all metadata, also embedded metadata. This
   will create an overhead of about 10% during storage. It wont give any
   overhead for read queries.

o. We will check if we can win back some of that 10% (some ideas are
   floating around)

o. If the embedded metadata can be left out of the quadruple then we
   should try to achieve that.

o. We wont try to remove the transaction from the decomposed tables yet.

People can followup development of the quadruple in the "quad" branch on
GNOME's git. This branch will likely soon be merged to "master".

The branch contains the support for the quadruple and, implemented on
top of that, support for backup. The branch doesn't yet implement named
graphs (the origin column is left empty for now).

Known concerns: Jamie's concern is a performance issue if we store both
embedded and non-embedded data in the quadruple. We will investigate
this performance issue and see if we can mitigate it.

Philip Van Hoof, freelance software developer
home: me at pvanhoof dot be 
gnome: pvanhoof at gnome dot org 

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