#2711 NORM Trial-3: implement real sorting in the datastore

[Zarro Boogs per Child]

#2711: implement real sorting in the datastore
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 Reporter:  tomeu      |       Owner:  bcsaller
     Type:  defect     |      Status:  new     
 Priority:  normal     |   Milestone:  Trial-3 
Component:  datastore  |     Version:          
 Keywords:             |    Verified:  0       
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 Now, the datastore service retrieves from the xapian index all the entries
 that match the filter, performs the requested sorting in-memory and then
 applies the paging limits.

 This affects performance quite badly when lots of results are stored in
 the index, and requires lots of memory. Both sorting and the paging limits
 should be applied by the xapian engine before returning the results.

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Ticket URL: <https://dev.laptop.org/ticket/2711>
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