code: function createIndex (model, column) {
      // build query
      var sql = 'ALTER TABLE '+quoteName(model.name)
      // column has unique index
      if (column.spec.unique) {
          sql = sql+' ADD '+createTableUniqueKey(column.name, column.spec)
      }
      // column has non-unique index
      else if (column.spec.index) {
          sql = sql+' ADD '+createTableKey(column.name, column.spec)
      }
      // reture sql string
      return sql
  }
docs: @function createIndex
  
  generate sql for ALTER TABLE ADD INDEX statement
  
  @param {ImmutableCoreModel} model
  @param {object} column
  
  @returns {string}
  
  @throws {Error}