feat(spanner/spannertest): support AVG aggregation function

spanner/spannertest/funcs.go

@@ -109,6 +109,43 @@ var aggregateFuncs = map[string]aggregateFunc{
 			return sum, typ, nil
 		},
 	},
+	"AVG": {
+		Eval: func(values []interface{}, typ spansql.Type) (interface{}, spansql.Type, error) {
+			if typ.Array || !(typ.Base == spansql.Int64 || typ.Base == spansql.Float64) {
+				return nil, spansql.Type{}, fmt.Errorf("AVG only supports arguments of INT64 or FLOAT64 type, not %s", typ.SQL())
+			}
+			if typ.Base == spansql.Int64 {
+				var sum int64
+				var n float64
+				for _, v := range values {
+					if v == nil {
+						continue
+					}
+					sum += v.(int64)
+					n++
+				}
+				if n == 0 {
+					// "Returns NULL if the input contains only NULLs".
+					return nil, typ, nil
+				}
+				return (float64(sum) / n), float64Type, nil
+			}
+			var sum float64
+			var n float64
+			for _, v := range values {
+				if v == nil {
+					continue
+				}
+				sum += v.(float64)
+				n++
+			}
+			if n == 0 {
+				// "Returns NULL if the input contains only NULLs".
+				return nil, typ, nil
+			}
+			return (sum / n), typ, nil
+		},
+	},
 }
 
 func evalMinMax(name string, isMin bool, values []interface{}, typ spansql.Type) (interface{}, spansql.Type, error) {

spanner/spannertest/integration_test.go

@@ -889,6 +889,13 @@ func TestIntegration_ReadsAndQueries(t *testing.T) {
 				{int64(1), int64(25)}, // Jack(ID=1, Tenure=10), Sam(ID=3, Tenure=9), George(ID=5, Tenure=6)
 			},
 		},
+		{
+			`SELECT AVG(Height) FROM Staff WHERE ID <= 2`,
+			nil,
+			[][]interface{}{
+				{float64(1.84)},
+			},
+		},
 		{
 			`SELECT MAX(Name) FROM Staff WHERE Name < @lim`,
 			map[string]interface{}{"lim": "Teal'c"},

spanner/spansql/keywords.go

@@ -130,6 +130,7 @@ var keywords = map[string]bool{
 var funcs = map[string]bool{
 	// Aggregate functions.
 	"ARRAY_AGG": true,
+	"AVG":       true,
 	"BIT_XOR":   true,
 	"COUNT":     true,
 	"MAX":       true,