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  1. Data Management
  2. DM-12823

Clarify how the Source table will be sharded in Qserv

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      DM Science

      Description

      A discussion with Fritz Mueller recently uncovered a (rather small) issue in our data model.

      The definition of the Source table in the DPDD (LSE-163) states that there is not required to be an Object for every Source:

      Name Type Unit Description
      objectId uint64   ID of the Object this Source was associated with, if any.

      In formal terms, this violates the Qserv sharding model for the DRP tables, in which the spatial sharding is "directed" by the ra/dec from the Object table, so that Sources and ForcedSources always end up in the same shard as their corresponding Object.

      There are several possible solutions to this. Two that occurred to us immediately are:

      1. Allow a fallback strategy in which orphaned Sources are sharded by their own ra/dec.
      2. Create a "fake Object" for each orphaned Source, with an appropriate value set in its flags attribute and with an ra/dec taken from the Source, and thereby enforce a requirement that, formally, every Source has an Object.

      Note that both of these have the same effect in terms of which shard an orphaned Source ends up in.

      From a database design point of view the second approach may be simpler, as it allows the statement "Object is the director table for Source" to have a more uniform meaning.

      Intuitively I'd expect orphaned Sources to be rare and usually of limited scientific interest (e.g., compared to the corresponding DIASource which would likely exist if it were a real astrophysical single-epoch detection), but the formal problem needs to have a solution documented.

      (There are similar but somewhat more complex questions associated with the DRP DIA* tables. I'll post separately about that.)

      I'm temporarily assigning this to myself as LSP Scientist, but ultimately this needs to be written up in an update to the database design.

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            • Assignee:
              ctslater Colin Slater
              Reporter:
              gpdf Gregory Dubois-Felsmann
              Watchers:
              Colin Slater, Fritz Mueller, Gregory Dubois-Felsmann, John Swinbank, Kian-Tat Lim, Robert Lupton, Zeljko Ivezic
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