rethinkdb 什么时候会返回一个游标 [英] When will rethinkdb return a cursor

问题描述

我注意到 r.table('xxx') 并不总是返回一个 cursor 但有时也只是直接返回 docs

1. 是 cursor 客户端的唯一实现还是 server 做了一些特殊的事情来执行与 cursor 相关的查询?

2. 如果它有与server相关的东西，它是什么，我什么时候会收到cursor

<块引用>

例如，我在查询中使用 skip 和 limit 指定结果偏移量和大小.服务器会返回 cursor 还是只返回结果 docs?

解决方案

当查询返回一个流时，驱动程序返回一个游标.

基本上当服务器产生一个流(一个延迟计算的序列)时，驱动程序将返回一个游标.当您从游标中获取行时，服务器将计算序列中的更多元素.

例如，当您运行 r.table('xxx') 时，您将返回一个游标.当您通过驱动程序请求文件时，服务器将从磁盘加载文件.

在 JavaScript 驱动程序中，当查询返回一个数组时，驱动程序将潜入一个模仿数组本身和 Array.prototype 之间的游标接口的对象.所以如果 query.run(...) 返回一个序列，你可以做

query.run(connection).then(function(result) {返回结果.toArray()}).then(函数(结果){//对结果做一些事情}).错误(函数(错误){//处理错误})

基本上，如果您不想考虑返回的是游标还是数组，您可以将其视为游标.

您可以在此处阅读有关流/游标的更多信息:
http://www.rethinkdb.com/docs/data-types/>

I notice that r.table('xxx') not always return a cursor but also sometimes just return docs directly

1. Is the cursor client side implementation only or there are some special things server did to perform queries associate with a cursor?

2. If it has somethings related to server, what is it and when will I receive a cursor

For example I specify result offset and size with skip and limit in the query. Will server return a cursor or just result docs?

解决方案

A driver returns a cursor when the query returns a stream.

Basically when the server produces a stream (a sequence that is lazily computed), the driver will return a cursor. As you fetch rows from the cursor, the server will compute more elements in the sequence.

For example, when you run r.table('xxx'), you will get back a cursor. The server will load the documents from disk as you request them with the driver.

In the JavaScript driver, when a query return an array, the driver will sneak an object that mimics the cursor interface between the arrray itself and Array.prototype. So if query.run(...) returns a sequence, you can just do

query.run(connection).then(function(result) {
    return result.toArray()
}).then(function(result) {
    // do something with result
}).error(function(err) {
    // handle err
})

Basically if you don't want to think if you are getting back a cursor or an array, you can just consider that it's a cursor.

You can read more about stream/cursor here:
http://www.rethinkdb.com/docs/data-types/

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