code: func (a *aggregationPushDownSolver) decompose(ctx sessionctx.Context, aggFunc *aggregation.AggFuncDesc, schema *expression.Schema) ([]*aggregation.AggFuncDesc, *expression.Schema) {
  	// Result is a slice because avg should be decomposed to sum and count. Currently we don't process this case.
  	result := []*aggregation.AggFuncDesc{aggFunc.Clone()}
  	for _, aggFunc := range result {
  		schema.Append(&expression.Column{
  			ColName:  model.NewCIStr(fmt.Sprintf("join_agg_%d", schema.Len())), // useless but for debug
  			UniqueID: ctx.GetSessionVars().AllocPlanColumnID(),
  			RetType:  aggFunc.RetTp,
  		})
  	}
  	aggFunc.Args = expression.Column2Exprs(schema.Columns[schema.Len()-len(result):])
  	aggFunc.Mode = aggregation.FinalMode
  	return result, schema
  }
docs: // decompose splits an aggregate function to two parts: a final mode function and a partial mode function. Currently
  // there are no differences between partial mode and complete mode, so we can confuse them.