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Fix an issue where paginating notifications could fail a GROUP BY test

Authored by epriestley on Feb 26 2021, 6:45 PM.
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F10864475: D21577.diff
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Ref T13623. When paginating notifications, we may currently construct a query which:

  • loads from non-unique rows; and
  • returns multiple results.

In particular, chronologicalKey isn't unique across the whole table (only for a given viewer). We can get away with this because no user-facing view of notifications is truly "every notification for every viewer" today.

One fix would be to implicitly force the paging query to include withUserPHIDs(viewerPHID), but puruse a slightly more general fix:

  • Load only unique stories.
  • Explictly limit the pagination subquery to one result.
Test Plan
  • Set page size to 1, inserted duplicate notifications of all stories for another user, clicked "Next", got the GROUP BY error.
  • Applied the "only load unique stories" part of the change, got a "expected one row" error instead.
  • Applied the "limit 1" part of the change, got a second page of notifications.

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