executor: support window function nth_value

executor/aggfuncs/builder.go

@@ -69,6 +69,8 @@ func BuildWindowFunctions(ctx sessionctx.Context, windowFuncDesc *aggregation.Ag
 		return buildFirstValue(windowFuncDesc, ordinal)
 	case ast.WindowFuncLastValue:
 		return buildLastValue(windowFuncDesc, ordinal)
+	case ast.WindowFuncNthValue:
+		return buildNthValue(windowFuncDesc, ordinal)
 	default:
 		return Build(ctx, windowFuncDesc, ordinal)
 	}
@@ -368,3 +370,13 @@ func buildLastValue(aggFuncDesc *aggregation.AggFuncDesc, ordinal int) AggFunc {
 	}
 	return &lastValue{baseAggFunc: base, tp: aggFuncDesc.RetTp}
 }
+
+func buildNthValue(aggFuncDesc *aggregation.AggFuncDesc, ordinal int) AggFunc {
+	base := baseAggFunc{
+		args:    aggFuncDesc.Args,
+		ordinal: ordinal,
+	}
+	// Already checked when building the function description.
+	nth, _, _ := expression.GetUint64FromConstant(aggFuncDesc.Args[1])
+	return &nthValue{baseAggFunc: base, tp: aggFuncDesc.RetTp, nth: nth}
+}

executor/aggfuncs/func_value.go

@@ -300,3 +300,50 @@ func (v *lastValue) AppendFinalResult2Chunk(sctx sessionctx.Context, pr PartialR
 	}
 	return nil
 }
+
+type nthValue struct {
+	baseAggFunc
+
+	tp  *types.FieldType
+	nth uint64
+}
+
+type partialResult4NthValue struct {
+	seenRows  uint64
+	evaluator valueEvaluator
+}
+
+func (v *nthValue) AllocPartialResult() PartialResult {
+	return PartialResult(&partialResult4NthValue{evaluator: buildValueEvaluator(v.tp)})
+}
+
+func (v *nthValue) ResetPartialResult(pr PartialResult) {
+	p := (*partialResult4NthValue)(pr)
+	p.seenRows = 0
+}
+
+func (v *nthValue) UpdatePartialResult(sctx sessionctx.Context, rowsInGroup []chunk.Row, pr PartialResult) error {
+	if v.nth == 0 {
+		return nil
+	}
+	p := (*partialResult4NthValue)(pr)
+	numRows := uint64(len(rowsInGroup))
+	if v.nth > p.seenRows && v.nth-p.seenRows <= numRows {
+		err := p.evaluator.evaluateRow(sctx, v.args[0], rowsInGroup[v.nth-p.seenRows-1])
+		if err != nil {
+			return err
+		}
+	}
+	p.seenRows += numRows
+	return nil
+}
+
+func (v *nthValue) AppendFinalResult2Chunk(sctx sessionctx.Context, pr PartialResult, chk *chunk.Chunk) error {
+	p := (*partialResult4NthValue)(pr)
+	if v.nth == 0 || p.seenRows < v.nth {
+		chk.AppendNull(v.ordinal)
+	} else {
+		p.evaluator.appendResult(chk, v.ordinal)
+	}
+	return nil
+}

executor/window_test.go

@@ -103,4 +103,13 @@ func (s *testSuite2) TestWindowFunctions(c *C) {
 	result = tk.MustQuery("select a, first_value(rand(0)) over(), last_value(rand(0)) over() from t")
 	result.Check(testkit.Rows("1 0.9451961492941164 0.05434383959970039", "1 0.9451961492941164 0.05434383959970039",
 		"2 0.9451961492941164 0.05434383959970039", "2 0.9451961492941164 0.05434383959970039"))
+
+	result = tk.MustQuery("select a, nth_value(a, null) over() from t")
+	result.Check(testkit.Rows("1 <nil>", "1 <nil>", "2 <nil>", "2 <nil>"))
+	result = tk.MustQuery("select a, nth_value(a, 1) over() from t")
+	result.Check(testkit.Rows("1 1", "1 1", "2 1", "2 1"))
+	result = tk.MustQuery("select a, nth_value(a, 4) over() from t")
+	result.Check(testkit.Rows("1 2", "1 2", "2 2", "2 2"))
+	result = tk.MustQuery("select a, nth_value(a, 5) over() from t")
+	result.Check(testkit.Rows("1 <nil>", "1 <nil>", "2 <nil>", "2 <nil>"))
 }

expression/aggregation/base_func.go

@@ -92,7 +92,7 @@ func (a *baseFuncDesc) typeInfer(ctx sessionctx.Context) {
 	case ast.AggFuncGroupConcat:
 		a.typeInfer4GroupConcat(ctx)
 	case ast.AggFuncMax, ast.AggFuncMin, ast.AggFuncFirstRow,
-		ast.WindowFuncFirstValue, ast.WindowFuncLastValue:
+		ast.WindowFuncFirstValue, ast.WindowFuncLastValue, ast.WindowFuncNthValue:
 		a.typeInfer4MaxMin(ctx)
 	case ast.AggFuncBitAnd, ast.AggFuncBitOr, ast.AggFuncBitXor:
 		a.typeInfer4BitFuncs(ctx)

expression/aggregation/window_func.go

@@ -28,6 +28,13 @@ type WindowFuncDesc struct {
 
 // NewWindowFuncDesc creates a window function signature descriptor.
 func NewWindowFuncDesc(ctx sessionctx.Context, name string, args []expression.Expression) *WindowFuncDesc {
+	if strings.ToLower(name) == ast.WindowFuncNthValue {
+		val, isNull, ok := expression.GetUint64FromConstant(args[1])
+		// nth_value does not allow `0`, but allows `null`.
+		if !ok || (val == 0 && !isNull) {
+			return nil
+		}
+	}
 	return &WindowFuncDesc{newBaseFuncDesc(ctx, name, args)}
 }
 

expression/util.go

@@ -20,6 +20,7 @@ import (
 	"unicode"
 
 	"github.com/pingcap/errors"
+	"github.com/pingcap/log"
 	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/mysql"
 	"github.com/pingcap/parser/opcode"
@@ -28,6 +29,7 @@ import (
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/types/parser_driver"
 	"github.com/pingcap/tidb/util/chunk"
+	"go.uber.org/zap"
 	"golang.org/x/tools/container/intsets"
 )
 
@@ -670,3 +672,34 @@ func RemoveDupExprs(ctx sessionctx.Context, exprs []Expression) []Expression {
 	}
 	return res
 }
+
+// GetUint64FromConstant gets a uint64 from constant expression.
+func GetUint64FromConstant(expr Expression) (uint64, bool, bool) {
+	con, ok := expr.(*Constant)
+	if !ok {
+		log.Warn("not a constant expression", zap.Any("value", expr))
+		return 0, false, false
+	}
+	dt := con.Value
+	if con.DeferredExpr != nil {
+		var err error
+		dt, err = con.DeferredExpr.Eval(chunk.Row{})
+		if err != nil {
+			log.Warn("eval deferred expr failed", zap.Error(err))
+			return 0, false, false
+		}
+	}
+	switch dt.Kind() {
+	case types.KindNull:
+		return 0, true, true
+	case types.KindInt64:
+		val := dt.GetInt64()
+		if val < 0 {
+			return 0, false, false
+		}
+		return uint64(val), false, true
+	case types.KindUint64:
+		return dt.GetUint64(), false, true
+	}
+	return 0, false, false
+}

planner/core/logical_plan_builder.go

@@ -2762,8 +2762,9 @@ func (b *PlanBuilder) buildProjectionForWindow(p LogicalPlan, expr *ast.WindowFu
 			return nil, nil, nil, nil, err
 		}
 		p = np
-		if col, ok := newArg.(*expression.Column); ok {
-			newArgList = append(newArgList, col)
+		switch newArg.(type) {
+		case *expression.Column, *expression.Constant:
+			newArgList = append(newArgList, newArg)
 			continue
 		}
 		proj.Exprs = append(proj.Exprs, newArg)
@@ -2948,6 +2949,9 @@ func (b *PlanBuilder) buildWindowFunction(p LogicalPlan, expr *ast.WindowFuncExp
 		return nil, err
 	}
 	desc := aggregation.NewWindowFuncDesc(b.ctx, expr.F, args)
+	if desc == nil {
+		return nil, ErrWrongArguments.GenWithStackByArgs(expr.F)
+	}
 	// TODO: Check if the function is aggregation function after we support more functions.
 	desc.WrapCastForAggArgs(b.ctx)
 	window := LogicalWindow{

planner/core/logical_plan_test.go

@@ -2195,6 +2195,14 @@ func (s *testPlanSuite) TestWindowFunction(c *C) {
 			sql:    "select row_number() over(rows between 1 preceding and 1 following) from t",
 			result: "TableReader(Table(t))->Window(row_number() over())->Projection",
 		},
+		{
+			sql:    "select nth_value(a, 1.0) over() from t",
+			result: "[planner:1210]Incorrect arguments to nth_value",
+		},
+		{
+			sql:    "select nth_value(a, 0) over() from t",
+			result: "[planner:1210]Incorrect arguments to nth_value",
+		},
 	}
 
 	s.Parser.EnableWindowFunc(true)