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SELECT p, array_agg(SELECT c FROM "MasterDetail"."Child_entity" c WHERE c."parentID" = p."ID") as c FROM "MasterDetail"."Parent_entity" p

Note, just replace array_agg with json_agg to get JSON.

And doing this the query iterates only once over the MasterDetail table ?

This particular query does a nested loop on the detail table, but basically yes. Only one iteration.