[Postgres] Add support for DISTINCT ON clause.

internal/jet/clause.go

@@ -18,8 +18,9 @@ type ClauseWithProjections interface {
 
 // ClauseSelect struct
 type ClauseSelect struct {
-	Distinct       bool
-	ProjectionList []Projection
+	Distinct          bool
+	DistinctOnColumns []ColumnExpression
+	ProjectionList    []Projection
 }
 
 // Projections returns list of projections for select clause
@@ -36,6 +37,12 @@ func (s *ClauseSelect) Serialize(statementType StatementType, out *SQLBuilder, o
 		out.WriteString("DISTINCT")
 	}
 
+	if len(s.DistinctOnColumns) > 0 {
+		out.WriteString("ON (")
+		SerializeColumnExpressions(s.DistinctOnColumns, statementType, out)
+		out.WriteByte(')')
+	}
+
 	if len(s.ProjectionList) == 0 {
 		panic("jet: SELECT clause has to have at least one projection")
 	}

postgres/select_statement.go

@@ -44,7 +44,7 @@ type SelectStatement interface {
 	jet.HasProjections
 	Expression
 
-	DISTINCT() SelectStatement
+	DISTINCT(on ...jet.ColumnExpression) SelectStatement
 	FROM(tables ...ReadableTable) SelectStatement
 	WHERE(expression BoolExpression) SelectStatement
 	GROUP_BY(groupByClauses ...GroupByClause) SelectStatement
@@ -104,8 +104,9 @@ type selectStatementImpl struct {
 	For     jet.ClauseFor
 }
 
-func (s *selectStatementImpl) DISTINCT() SelectStatement {
+func (s *selectStatementImpl) DISTINCT(on ...jet.ColumnExpression) SelectStatement {
 	s.Select.Distinct = true
+	s.Select.DistinctOnColumns = on
 	return s
 }
 

tests/postgres/select_test.go

@@ -48,6 +48,63 @@ WHERE actor.actor_id = 2;
 	requireLogged(t, query)
 }
 
+func TestSelectDistinctOn(t *testing.T) {
+
+	stmt := SELECT(
+		Rental.StaffID,
+		Rental.CustomerID,
+		Rental.RentalID,
+	).DISTINCT(
+		Rental.StaffID,
+		Rental.CustomerID,
+	).FROM(
+		Rental,
+	).WHERE(
+		Rental.CustomerID.LT(Int(2)),
+	).ORDER_BY(
+		Rental.StaffID.ASC(),
+		Rental.CustomerID.ASC(),
+		Rental.RentalID.ASC(),
+	)
+
+	testutils.AssertDebugStatementSql(t, stmt, `
+SELECT DISTINCT ON (rental.staff_id, rental.customer_id) rental.staff_id AS "rental.staff_id",
+     rental.customer_id AS "rental.customer_id",
+     rental.rental_id AS "rental.rental_id"
+FROM dvds.rental
+WHERE rental.customer_id < 2
+ORDER BY rental.staff_id ASC, rental.customer_id ASC, rental.rental_id ASC;
+`)
+
+	var dest []model.Rental
+
+	err := stmt.Query(db, &dest)
+	require.NoError(t, err)
+
+	testutils.AssertJSON(t, dest, `
+[
+	{
+		"RentalID": 573,
+		"RentalDate": "0001-01-01T00:00:00Z",
+		"InventoryID": 0,
+		"CustomerID": 1,
+		"ReturnDate": null,
+		"StaffID": 1,
+		"LastUpdate": "0001-01-01T00:00:00Z"
+	},
+	{
+		"RentalID": 76,
+		"RentalDate": "0001-01-01T00:00:00Z",
+		"InventoryID": 0,
+		"CustomerID": 1,
+		"ReturnDate": null,
+		"StaffID": 2,
+		"LastUpdate": "0001-01-01T00:00:00Z"
+	}
+]
+`)
+}
+
 func TestClassicSelect(t *testing.T) {
 	expectedSQL := `
 SELECT payment.payment_id AS "payment.payment_id",