code: func (o *outerJoinEliminator) extractInnerJoinKeys(join *LogicalJoin, innerChildIdx int) *expression.Schema {
  	joinKeys := make([]*expression.Column, 0, len(join.EqualConditions))
  	for _, eqCond := range join.EqualConditions {
  		joinKeys = append(joinKeys, eqCond.GetArgs()[innerChildIdx].(*expression.Column))
  	}
  	return expression.NewSchema(joinKeys...)
  }
docs: // extract join keys as a schema for inner child of a outer join