code: func (a *aggPushDownSolver) decompose(aggFunc expression.AggregationFunction, schema expression.Schema, id string) ([]expression.AggregationFunction, expression.Schema) {
  	// Result is a slice because avg should be decomposed to sum and count. Currently we don't process this case.
  	result := []expression.AggregationFunction{aggFunc.Clone()}
  	for _, aggFunc := range result {
  		schema.Append(&expression.Column{
  			ColName:  model.NewCIStr(fmt.Sprintf("join_agg_%d", schema.Len())), // useless but for debug
  			FromID:   id,
  			Position: len(schema.Columns),
  			RetType:  aggFunc.GetType(),
  		})
  	}
  	aggFunc.SetArgs(expression.Column2Exprs(schema.Columns[len(schema.Columns)-len(result):]))
  	aggFunc.SetMode(expression.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.