code: func (p *LogicalSelection) checkMaxOneRowCond(unique expression.Expression, constOrCorCol expression.Expression) bool {
  	col, ok := unique.(*expression.Column)
  	if !ok {
  		return false
  	}
  	if !p.children[0].Schema().IsUniqueKey(col) {
  		return false
  	}
  	_, okCon := constOrCorCol.(*expression.Constant)
  	if okCon {
  		return true
  	}
  	_, okCorCol := constOrCorCol.(*expression.CorrelatedColumn)
  	return okCorCol
  }
docs: // If a condition is the form of (uniqueKey = constant) or (uniqueKey = Correlated column), it returns at most one row.
  // This function will check it.