MongoDB 索引 (ensureIndex)

Question

例如我收集了这样的文档 foo:

{"tag_cloud":[{"value":"games", "count":10}, {"value":"girls", "count":500}]}
{"tag_cloud":[{"value":"games", "count":5}, {"value":"stack", "count":100}]}
{"tag_cloud":[{"value":"mongodb", "count":1000}, {"value":"thread", "count":15}]}

两种索引有什么区别:

1. ensureIndex({"tag_cloud"})
2. ensureIndex({"tag_cloud.value"}); ensureIndex({"tag_cloud.count"})

在请求的上下文中:

db.foo.find({"tag_cloud.value":"games"}).sort({"tag_cloud.count":-1});

谢谢!!!

Answer 1

MongoDB 每次查询一次只能使用一个索引，因此您创建两个索引的建议将行不通。相反，我会创建一个复合索引:

> db.foo.ensureIndex({"tag_cloud.value": 1, "tag_cloud.count": -1})

此索引既可用于过滤到您要考虑的特定元素，也可用于按降序排序返回:

> db.foo.find({"tag_cloud.value":"games"}).sort({"tag_cloud.count":-1}).explain()
{
    "cursor" : "BtreeCursor tag_cloud.value_1_tag_cloud.count_-1",
    "isMultiKey" : true,
    "n" : NumberLong(2),
    "nscannedObjects" : NumberLong(2),
    "nscanned" : NumberLong(2),
    ...