code: function createTableKeyMulti (spec) {
      // require spec to be object
      assert.equal(typeof spec, 'object', 'spec must be object')
      // require columns to be array
      assert.ok(Array.isArray(spec.columns), 'index columns must be array')
      // return key sql
      return 'KEY ('+_.map(spec.columns, quoteName).join(', ')+')'
  }
docs: @function createTableKeyMulti
  
  generate multi-column non-unique index sql for CREATE TABLE
  
  @param {object} spec
  
  @returns {string}
  
  @throws {Error}