Fix Postgres query handling null values for smallint (#36648)

* Fix Postgres query handling null values for smallint

* Fix converting to int16

pkg/tsdb/postgres/postgres.go

@@ -200,5 +200,25 @@ func (t *postgresQueryResultTransformer) GetConverterList() []sqlutil.StringConv
 				},
 			},
 		},
+		{
+			Name:           "handle INT2",
+			InputScanKind:  reflect.Interface,
+			InputTypeName:  "INT2",
+			ConversionFunc: func(in *string) (*string, error) { return in, nil },
+			Replacer: &sqlutil.StringFieldReplacer{
+				OutputFieldType: data.FieldTypeNullableInt16,
+				ReplaceFunc: func(in *string) (interface{}, error) {
+					if in == nil {
+						return nil, nil
+					}
+					i64, err := strconv.ParseInt(*in, 10, 16)
+					if err != nil {
+						return nil, err
+					}
+					v := int16(i64)
+					return &v, nil
+				},
+			},
+		},
 	}
 }

pkg/tsdb/postgres/postgres_test.go

@@ -143,7 +143,7 @@ func TestGenerateConnectionString(t *testing.T) {
 }
 
 // To run this test, set runPostgresTests=true
-// Or from the commandline: GRAFANA_TEST_DB=postgres go test -v ./pkg/tsdb/postgres
+// Or from the commandline: GRAFANA_TEST_DB=postgres go test -tags=integration -v ./pkg/tsdb/postgres
 // The tests require a PostgreSQL db named grafanadstest and a user/password grafanatest/grafanatest!
 // Use the docker/blocks/postgres_tests/docker-compose.yaml to spin up a
 // preconfigured Postgres server suitable for running these tests.
@@ -214,7 +214,8 @@ func TestPostgres(t *testing.T) {
 				c13_time time without time zone,
 				c14_timetz time with time zone,
 				time date,
-				c15_interval interval
+				c15_interval interval,
+				c16_smallint smallint
 			);
 		`
 		_, err := sess.Exec(sql)
@@ -226,7 +227,8 @@ func TestPostgres(t *testing.T) {
 				4.5,6.7,1.1,1.2,
 				'char10','varchar10','text',
 
-				now(),now(),now(),now(),now(),now(),'15m'::interval
+				now(),now(),now(),now(),now(),now(),'15m'::interval,
+				null
 			);
 		`
 		_, err = sess.Exec(sql)
@@ -252,9 +254,9 @@ func TestPostgres(t *testing.T) {
 
 			frames, _ := queryResult.Dataframes.Decoded()
 			require.Len(t, frames, 1)
-			require.Len(t, frames[0].Fields, 17)
+			require.Len(t, frames[0].Fields, 18)
 
-			require.Equal(t, int16(1), frames[0].Fields[0].At(0).(int16))
+			require.Equal(t, int16(1), *frames[0].Fields[0].At(0).(*int16))
 			require.Equal(t, int32(2), *frames[0].Fields[1].At(0).(*int32))
 			require.Equal(t, int64(3), *frames[0].Fields[2].At(0).(*int64))
 
@@ -280,6 +282,7 @@ func TestPostgres(t *testing.T) {
 			_, ok = frames[0].Fields[15].At(0).(*time.Time)
 			require.True(t, ok)
 			require.Equal(t, "00:15:00", *frames[0].Fields[16].At(0).(*string))
+			require.Nil(t, frames[0].Fields[17].At(0))
 		})
 	})