Can I use… with recursive (… union [distinct] …)

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Union [distinct] within with recursive can be used to omit duplicate rows and thus preventing infinite loops.

WITH RECURSIVE path (a, b) AS (
  SELECT edges.* FROM edges
                WHERE a = 1
  SELECT edges.* FROM edges
                 JOIN path 
                   ON edges.a = path.b
  FROM path

The example properly terminates even if the graph stored in the edges table has cycles.

Normative References

The with recursive clause is defined in ISO/IEC 9075:2016-2 as part of optional feature T131, “Recursive query”. Union [distinct] belongs to mandatory feature E071-01.

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