From 245c1ee3d3afbf22797ab0ea85b227796efa934b Mon Sep 17 00:00:00 2001
From: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>
Date: Fri, 21 Oct 2022 10:03:43 -0400
Subject: [PATCH] Chore: Add `last` downsampling function to Resample
 expression (#57379)

---
 pkg/expr/mathexp/resample.go             |  4 +++
 pkg/expr/mathexp/resample_test.go        | 32 ++++++++++++++++++++++++
 public/app/features/expressions/types.ts |  1 +
 3 files changed, 37 insertions(+)

diff --git a/pkg/expr/mathexp/resample.go b/pkg/expr/mathexp/resample.go
index 13cd4b0914a..f6239be59f9 100644
--- a/pkg/expr/mathexp/resample.go
+++ b/pkg/expr/mathexp/resample.go
@@ -54,6 +54,8 @@ func (s Series) Resample(refID string, interval time.Duration, downsampler strin
 			default:
 				return s, fmt.Errorf("upsampling %v not implemented", upsampler)
 			}
+		} else if len(vals) == 1 {
+			value = vals[0]
 		} else { // downsampling
 			fVec := data.NewField("", s.GetLabels(), vals)
 			ff := Float64Field(*fVec)
@@ -67,6 +69,8 @@ func (s Series) Resample(refID string, interval time.Duration, downsampler strin
 				tmp = Min(&ff)
 			case "max":
 				tmp = Max(&ff)
+			case "last":
+				tmp = Last(&ff)
 			default:
 				return s, fmt.Errorf("downsampling %v not implemented", downsampler)
 			}
diff --git a/pkg/expr/mathexp/resample_test.go b/pkg/expr/mathexp/resample_test.go
index 352d979ed6c..a09ae962266 100644
--- a/pkg/expr/mathexp/resample_test.go
+++ b/pkg/expr/mathexp/resample_test.go
@@ -245,6 +245,38 @@ func TestResampleSeries(t *testing.T) {
 				time.Unix(10, 0), nil,
 			}),
 		},
+		{
+			name:        "resample series: downsampling (last / pad )",
+			interval:    time.Second * 3,
+			downsampler: "last",
+			upsampler:   "pad",
+			timeRange: backend.TimeRange{
+				From: time.Unix(0, 0),
+				To:   time.Unix(11, 0),
+			},
+			seriesToResample: makeSeries("", nil, tp{
+				time.Unix(0, 0), float64Pointer(0),
+			}, tp{
+				time.Unix(2, 0), float64Pointer(2),
+			}, tp{
+				time.Unix(4, 0), float64Pointer(3),
+			}, tp{
+				time.Unix(6, 0), float64Pointer(4),
+			}, tp{
+				time.Unix(8, 0), float64Pointer(0),
+			}, tp{
+				time.Unix(10, 0), float64Pointer(1),
+			}),
+			series: makeSeries("", nil, tp{
+				time.Unix(0, 0), float64Pointer(0),
+			}, tp{
+				time.Unix(3, 0), float64Pointer(2),
+			}, tp{
+				time.Unix(6, 0), float64Pointer(4),
+			}, tp{
+				time.Unix(9, 0), float64Pointer(0),
+			}),
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
diff --git a/public/app/features/expressions/types.ts b/public/app/features/expressions/types.ts
index 0fbbff79d7e..b67b9fe6a81 100644
--- a/public/app/features/expressions/types.ts
+++ b/public/app/features/expressions/types.ts
@@ -75,6 +75,7 @@ export const reducerMode: Array<SelectableValue<ReducerMode>> = [
 ];
 
 export const downsamplingTypes: Array<SelectableValue<string>> = [
+  { value: ReducerID.last, label: 'Last', description: 'Fill with the last value' },
   { value: ReducerID.min, label: 'Min', description: 'Fill with the minimum value' },
   { value: ReducerID.max, label: 'Max', description: 'Fill with the maximum value' },
   { value: ReducerID.mean, label: 'Mean', description: 'Fill with the average value' },