Prompted by this thread at Project Chat,
   https://www.wikidata.org/wiki/Wikidata:Project_chat#Identical_data_sets

here's a query to find multiple humans with nationality:Greece that have
the same day of birth and day of death:
   http://tinyurl.com/ow6lpen
It produces one pair, and executes in about 0.6 seconds.

Here's a query to try to add item numbers and labels to the previous search:
   http://tinyurl.com/ovjwzc9

It *just* completes, taking just over 60 seconds to execute.

(Please don't merge the two items yet, because that will destroy the
example).

Analogous queries with lookups for France (71 apparent sets of
duplicates), UK (32), and Italy(14) fail to complete.


Two questions therefore:
(1)  Why are the two queries taking such different times to run ?
(2)  Is there a good way to rewrite the second to make it faster ?


Obviously the second query as written at the moment involves a
sub-query, which inevitably must make it a bit slower -- but given the
solution set of the sub-query only has two rows, and an exact date for a
given property ought to be a fairly quick key to look up, why is the
second query taking 100 times longer than the first ?

And is there a better way I should be doing this, since the query does
appear to be producing useful real matches ?

Thanks,

    James.


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