女主宣言
某日公司研发同事突然反馈MongoDB的某个分片集合按照分片键无法检索到对应内容，但是按照_id却可以检索到对应内容，一瓜未平，一瓜又起，另一个研发同事反馈该集合唯一索引失效了，存在重复数据，这简直是比对面老王有女朋友还让人震惊的事情。接下来我会给大家复现这个现象并说明具体的原理。
1
问题现象模拟
因为涉及到公司的一些信息安全政策故对集合的字段名进行替换，数据里面敏感信息进行值替换，但是不影响最终结果。
MongoDB版本： mongos 4.0.12 社区版本
数据库:mdb_test
哈希分片集合:mdb_test_coll
分片键:key3
集合索引信息如下，key3是分片键 ，还有一个联合唯一索引key3_1_key1_1_key2_1_key4_1。
db.mdb_test_coll.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "mdb_test.mdb_test_coll"
},
{
"v" : 2,
"key" : {
"key3" : "hashed"
},
"name" : "key3_hashed",
"ns" : "mdb_test.mdb_test_coll"
},
{
"v" : 2,
"key" : {
"key2" : 1
},
"name" : "key2_1",
"background" : true,
"ns" : "mdb_test.mdb_test_coll"
},
{
"v" : 2,
"unique" : true,
"key" : {
"key3" : 1,
"key1" : 1,
"key2" : 1,
"key4" : 1
},
"name" : "key3_1_key1_1_key2_1_key4_1",
"background" : true,
"ns" : "mdb_test.mdb_test_coll"
}
]
模拟数据
注:在使用下面数据的时候建议将每一条都调整到一行进行插入，每条记录的_id是不一样的。
{
"_id" : ObjectId("606ed113137fa535ac53d3ee"),
"key1" : null,
"key2" : 1033072826,
"key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
{
"_id" : ObjectId("5fb2df3b32b0f42dced04ea7"),
"key1" : null,
"key2" : 2100000000,
"key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
{
"_id" : ObjectId("5fb2df3b32b0f42dced01127"),
"key1" : null,
"key2" : 2100000000,
"key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
{
"_id" : ObjectId("5fb2df3b32b0f42dced01130"),
"key1" : null,
"key2" : 2100000000,
"key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
操作步骤
先模拟研发反馈的无法通过分片键查询到数据，但是可以根据_id查询到数据。将模拟数据第一条插入集合中。
mongos> db.mdb_test_coll.find()
mongos> db.mdb_test_coll.save({
... "_id" : ObjectId("606ed113137fa535ac53d3ee"),
... "key1" : null,
... "key2" : 1033072826,
... "key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA=="),
... "key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
... })
WriteResult({
"nMatched" : 0,
"nUpserted" : 1,
"nModified" : 0,
"_id" : ObjectId("606ed113137fa535ac53d3ee")
})
mongos> db.mdb_test_coll.find()
{ "_id" : ObjectId("606ed113137fa535ac53d3ee"), "key1" : null, "key2" : 1033072826, "key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA=="), "key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl" }
#按照_id查询可以得到数据，但是将条件换成分片键或者带分片键条件的联合查询条件都无法获得任何结果。
mongos> db.mdb_test_coll.find({"_id" : ObjectId("606ed113137fa535ac53d3ee")})
{ "_id" : ObjectId("606ed113137fa535ac53d3ee"), "key1" : null, "key2" : 1033072826, "key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA=="), "key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl" }
mongos> db.mdb_test_coll.find({"key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA=="),"key2" : 1033072826})
mongos> db.mdb_test_coll.find({"key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA==")})
mongos> db.mdb_test_coll.find({"key2" : 1033072826,"key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA==")})
再来模拟一下研发反馈的唯一索引失效的现象。将模拟数据第二,三条插入集合中，请注意模拟数据二三两条除_id不一致，其余字段值都是相同的。
mongos> db.mdb_test_coll.save({
... "_id" : ObjectId("5fb2df3b32b0f42dced04ea7"),
... "key1" : null,
... "key2" : 2100000000,
... "key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
... "key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
... })
WriteResult({
"nMatched" : 0,
"nUpserted" : 1,
"nModified" : 0,
"_id" : ObjectId("5fb2df3b32b0f42dced04ea7")
})
mongos> db.mdb_test_coll.save({
... "_id" : ObjectId("5fb2df3b32b0f42dced01127"),
... "key1" : null,
... "key2" : 2100000000,
... "key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
... "key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
... })
WriteResult({
"nMatched" : 0,
"nUpserted" : 1,
"nModified" : 0,
"_id" : ObjectId("5fb2df3b32b0f42dced01127")
})
mongos> db.mdb_test_coll.find().pretty()
{
"_id" : ObjectId("606ed113137fa535ac53d3ee"),
"key1" : null,
"key2" : 1033072826,
"key3" : BinData(5,"N0YSB7m2iXUEzQnFsTQWPA=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
{
"_id" : ObjectId("5fb2df3b32b0f42dced04ea7"),
"key1" : null,
"key2" : 2100000000,
"key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
{
"_id" : ObjectId("5fb2df3b32b0f42dced01127"),
"key1" : null,
"key2" : 2100000000,
"key3" : BinData(5,"AnQTYWNGHKoj4xx+KTjNxQ=="),
"key4" : "hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hls/hl"
}
可以看见连续插入两条唯一字段值相同的记录，而且并没有触发唯一约束的，并且值是真实插入到集合中了。
2
原理分析
先来说一下问题现象2的根本原因，之所以可以插入重复数据而没有报错是因为设置了failIndexKeyTooLong，先来看一下官方文档中failIndexKeyTooLong这个参数定义
简单总结就是这个参数仅对mongod节点有效，并且这个参数主要作用就是如果当索引条目超过Index Key Length Limit 后的动作。默认为True也就是如果索引条目超过1024字节，报错给客户端。也可以设置为false，如果设置为False 超过1024字节的数据是可以插入到集合里面，但是不会在对应的索引里面，所以在查询的时候如果使用了对应索引，该索引并不会返回完整的数据给客户端(这块下面详细解释一下)，这个参数是可以动态调整的。
动态调整语句如下:
db.adminCommand( { setParameter: 1, failIndexKeyTooLong: false } )
这里再解释一下什么是Index Key Limit， 也就是上面提到的Index Key Length Limit。看一下官方文档对这个参数的定义：
一句概括就是索引条目必须小于1024字节
举个????
将failIndexKeyTooLong设置为true，然后插入数据。可以看见当设置为true的时候，插入模拟数据第四条是报错的。因为有一个唯一联合索引就是这四个字段组成的。
再来看一下failIndexKeyTooLong设置为false，当设置为false的时候是可以插入模拟数据第四条，并没有报错，而且也没有触发唯一约束，但是按照key3或者key3+key2联合条件查询不到数据。但是按照_id或者key2(最后插入的数据也是除了_id不同，别的字段值都相同)单独去匹配可以获得数据。
实际上到这里已经解释清了研发反馈的所有问题，在failIndexKeyTooLong设置为false的情况下，按照key3以及key3+key2联合去检索的时候实际使用的都是联合唯一索引，但是实际上因为索引条目超过1024字节，故没有录入到索引中同时也没有触发唯一索引约束，所以是查询不到数据的。但是按照_id或者key2单条件去检索都是使用了各自的单独索引， _id以及key2的索引条目在本案例中都是没有超过1024字节，故是可以检索到对应的结果的。实际key3如果单条件查询的时候强制指定key3_hashed也是可以查询到对应结果的，因为按照当前的已知的hash函数处理过后的结果都没有超过1024字节的。(这块留给爱学习的小伙伴一个疑问，是否还有另外的方法让本案例复现，如果所有的索引条目都没有超过1024字节，但是唯一索引仍然失效)
3
总结
对于研发同事来说，如果真是存在本案例情形，建议使用强制指定索引或者另外的索引去检索。同时也要反思一下为什么一个索引条目会超过1024字节，那么长的索引真的有必要存在么？对于DBA来说这个参数日常是否开启还是应该与研发一起商议在做决断。
实际从接到反馈到最后解决，总结就是可能日常很复杂的问题，到最后发现原理越简单，建议多读读官方文档。
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