func (af *aggFunction) getStreamedContext() *ast.AggEvaluateContext {
	if af.streamCtx == nil {
		af.streamCtx = &ast.AggEvaluateContext{}
		if af.Distinct {
			af.streamCtx.DistinctChecker = distinct.CreateDistinctChecker()
		}
	}
	return af.streamCtx
}

func (af *aggFunction) getContext(groupKey []byte) *ast.AggEvaluateContext {
	ctx, ok := af.resultMapper[string(groupKey)]
	if !ok {
		ctx = &ast.AggEvaluateContext{}
		if af.Distinct {
			ctx.DistinctChecker = distinct.CreateDistinctChecker()
		}
		af.resultMapper[string(groupKey)] = ctx
	}
	return ctx
}

// GetContext gets aggregate evaluation context for the current group.
// If it is nil, add a new context into contextPerGroupMap.
func (n *AggregateFuncExpr) GetContext() *AggEvaluateContext {
	if n.contextPerGroupMap == nil {
		n.contextPerGroupMap = make(map[string](*AggEvaluateContext))
	}
	if _, ok := n.contextPerGroupMap[n.CurrentGroup]; !ok {
		c := &AggEvaluateContext{}
		if n.Distinct {
			c.distinctChecker = distinct.CreateDistinctChecker()
		}
		n.contextPerGroupMap[n.CurrentGroup] = c
	}
	return n.contextPerGroupMap[n.CurrentGroup]
}

// Next implements Executor Next interface.
func (e *DistinctExec) Next() (*Row, error) {
	if e.checker == nil {
		e.checker = distinct.CreateDistinctChecker()
	}
	for {
		row, err := e.Src.Next()
		if err != nil {
			return nil, errors.Trace(err)
		}
		if row == nil {
			return nil, nil
		}
		ok, err := e.checker.Check(types.DatumsToInterfaces(row.Data))
		if err != nil {
			return nil, errors.Trace(err)
		}
		if !ok {
			continue
		}
		return row, nil
	}
}