How to group timestamps into islands (based on arbitrary gap)?

Question

Consider this list of dates as timestamptz:

I grouped the dates by hand using colors: every group is separated from the next by a gap of at least 2

Answer 1

This would do it:

SELECT done, count(*) FILTER (WHERE step) OVER (ORDER BY done) AS grp
FROM  (
   SELECT done
       , (lag(done) OVER (ORDER BY done) <= done - interval '2 min') AS step
   FROM   tbl
   ) sub
ORDER  BY done;

The subquery sub records step as true if the previous row is at least 2 min away - sorted by the timestamp column done itself in this case.

The outer query adds a rolling count of steps, effectively the group number (grp) - combining the aggregate FILTER clause with another window function.

db<>fiddle here

Related:

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About the aggregate FILTER clause:

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