code: func (af *aggFunction) CreateContext(sc *stmtctx.StatementContext) *AggEvaluateContext {
  	evalCtx := &AggEvaluateContext{}
  	if af.HasDistinct {
  		evalCtx.DistinctChecker = createDistinctChecker(sc)
  	}
  	return evalCtx
  }
docs: // CreateContext implements Aggregation interface.