code: function requireValidSchema (schema) {
      // throw error on invalid schema object
      assert.equal(typeof schema, 'object', 'schema must be object')
      assert.equal(typeof schema.columns, 'object', 'schema columns must be object')
      assert.ok(Array.isArray(schema.indexes), 'schema indexes must be array')
      // require validated schema
      return schema
  }
docs: @function requireValidSchema
  
  validate schema argument
  
  @param {object} schema
  
  @returns {object}
  
  @throws {Error}