2012/12/19 Martynas Jusevičius
<martynas(a)graphity.org>
Hey wikidatians,
occasionally checking threads in this list like the current one, I get
a mixed feeling: on one hand, it is sad to see the efforts and
resources waisted as Wikidata tries to reinvent RDF, and now also
triplestore design as well as XSD datatypes. What's next, WikiQL
instead of SPARQL?
On the other hand, it feels reassuring as I was right to predict this:
On Wed, Dec 19, 2012 at 4:11 PM, Daniel Kinzler
<daniel.kinzler(a)wikimedia.de> wrote:
On 19.12.2012 14:34, Friedrich Röhrs wrote:
Hi,
Sorry for my ignorance, if this is common knowledge: What is the use case for
sorting millions of different measures from different objects?
Finding all cities with more than 100000 inhabitants requires the database to
look through all values for the property "population" (or even all properties
with countable values, depending on implementation an query planning), compare
each value with "100000" and return those with a greater value. To speed this
up, an index sorted by this value would be needed.
For cars there could be entries by the
manufacturer, by some
car-testing magazine, etc. I don't see how this could be adequatly
represented/sorted by a database only query.
If this cannot be done adequatly on the database level, then it cannot be done
efficiently, which means we will not allow it. So our task is to come up with an
architecture that does allow this.
(One way to allow "scripted" queries like this to run efficiently is to do
this
in a massively parallel way, using a map/reduce framework. But that's also not
trivial, and would require a whole new server infrastructure).
If however this is necessary, i still don't
understand why it must affect the
datavalue structure. If a index is necessary it could be done over a serialized
representation of the value.
"Serialized" can mean a lot of things, but an index on some data blob is only
useful for exact matches, it can not be used for greater/lesser queries. We need
to map our values to scalar data types the database can understand directly, and
use for indexing.
This needs to be done anyway, since the values
are
saved at a specific unit (which is just a wikidata item). To compare them on a
database level they must all be saved at the same unit, or some sort of
procedure must be used to compare them (or am i missing something again?).
If they measure the same dimension, they should be saved using the same unit
(probably the SI base unit for that dimension). Saving values using different
units would make it impossible to run efficient queries against these values,
thereby defying one of the major reasons for Wikidata's existance. I don't see a
way around this.
-- daniel
--
Daniel Kinzler, Softwarearchitekt
Wikimedia Deutschland - Gesellschaft zur Förderung Freien Wissens e. V.
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