RETURNING docs

Author: Anton Kovalevsky

modules/docs/src/main/paradox/reference/Identifiers.md

@@ -1,2 +1,29 @@
 # Identifiers
 
+Please visit PostgreSQL documentation to familiarize with `RETURNING` syntax: [Returning Data From Modified Rows](https://www.postgresql.org/docs/9.5/dml-returning.html)
+
+## Returning identifiers
+
+```
+val savePet: Query[Pet, Int] =
+  sql"""
+    INSERT INTO pets (
+    VALUES ${petCodec.values}
+    RETURNING id;
+  """.query(int4)
+```
+
+## Returning entities
+
+It's reasonable to return just an identifier in case there are no other database-managed columns.
+Let's assume we have five columns: `id`, `name`, `colour`, `created_at`, `created_by` where `id`, `created_at`, `created_by` are not part of initial `NewPet` entity.
+That means you need to query whole entity with RETURNING expression.
+```
+val savePet: Query[NewPet, PersistedPet] =
+  sql"""
+    INSERT INTO #$PETS_TABLE_NAME (name, colour)
+    VALUES ${newPetCodec.values}
+    RETURNING id, name, colour, created_at, created_by;
+  """.query(persistedPetCodec)
+```
+Please note that you might want to write down all column names (not just `*`) to decouple entity and database representation.