Mongo $存在查询未返回正确的文档 [英] Mongo $exists query does not return correct documents

问题描述

老实说，我什至不知道这怎么可能:

Honestly, I don't understand how this is even possible:

> db.ts.find({"bcoded_metadata" : { "$exists" : true} } ).count()
199049
> db.ts.find({"bcoded_metadata" : { "$exists" : false} } ).count()
0
> db.ts.count()
2507873

我认为第一和第二个查询的总和必须等于第三个.

I think sum of the first and second queries must be equal to the third.

我需要从集合中选择所有不存在"bcoded_metadata"但查询不返回任何内容的元素. 当我在简单的python脚本中迭代此集合并手动检查是否存在"bcoded_metadata"时，一切正常.

I need to select from the collection all elements for which "bcoded_metadata" does not exist but the query returns nothing. When I iterate this collection in the simple python script and manually check whether "bcoded_metadata" exists everything works as expected.

from pymongo import Connection
connection = Connection('127.0.0.1', 27017)
db = connection.data
c = 0
for item in db.ts.find():
    if not "bcoded_metadata" in item.keys():
            c+= 1
print c



python test.py 
2308824

这是正确的答案.

问题根源在哪里?

索引:

> db.ts.getIndexes();
[
        {
                "name" : "_id_",
                "ns" : "data.ts",
                "key" : {
                        "_id" : 1
                },
                "v" : 0
        },
        {
                "_id" : ObjectId("4f3c299b4c4a5ccfddbe4069"),
                "ns" : "data.ts",
                "key" : {
                        "last_seen" : 1
                },
                "name" : "last_seen_1",
                "v" : 0
        },
        {
                "_id" : ObjectId("4f3c2cef4c4a5ccfddbe406a"),
                "ns" : "data.ts",
                "key" : {
                        "attempts" : -1
                },
                "name" : "attempts_-1",
                "v" : 0
        },
        {
                "_id" : ObjectId("4f4279ed6aca13be31acbe6d"),
                "ns" : "data.ts",
                "key" : {
                        "bcoded_metadata" : 1
                },
                "name" : "bcoded_metadata_1",
                "sparse" : true,
                "v" : 0
        }
]

推荐答案

这是因为您对bcoded_metadata使用了稀疏索引.如果bcoded_metadata上的索引稀疏，则该索引将不包含没有字段bcoded_metadata的文档.没有bcoded_metadata字段的文档不是原始查询的一部分，因此"count"将返回0.

This is because you use a sparse index for bcoded_metadata. If you have a sparse index on bcoded_metadata, then the index will not contain the documents that don't have the field bcoded_metadata. The documents without the bcoded_metadata field are not part of your original query, and hence "count" will return 0.

如果仅运行查找:db.ts.find({"bcoded_metadata" : { "$exists" : false } })，则也不会得到任何结果.您可以使用非稀疏索引，也可以使用db.ts.count();进行全计数并减去db.ts.find({"bcoded_metadata" : { "$exists" : true } })结果的结果.

If you run just the find: db.ts.find({"bcoded_metadata" : { "$exists" : false } }) then you won't get any results either. You can either use a non-sparse index, or do a full count with db.ts.count(); and subtract the result of db.ts.find({"bcoded_metadata" : { "$exists" : true } }) result.

有一个JIRA票证可以解释得更多，如果MongoDB为此获取错误/警告消息，可以进行跟踪:

There is a JIRA ticket that explains it a bit more, and can be tracked in case MongoDB acquires an error/warning message for this: https://jira.mongodb.org/browse/SERVER-3918

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