从5.0开始,还新增了一批lock-free read 操作,这些操作在其他操作持有同collection的排他写锁时也不会被阻塞,如find、count、distinct、aggregate、listCollections、listIndexes等,其中aggregate中包含对collection的写入时,会持有collection的意向排他锁。 以上资源层级自上而下优先级依次降低。为了防止出现死锁,一般而言在对...
1 just to make sure the $lookup is stopped fast */ { "$project" : { "_id" : 1 } } ] } , } match = { "$match" : { "_tmp.found.0" : { "$exists" : false } } } unset = { "$unset" : "_tmp" } out = { "$out" : "result" } db.Dual.aggregate( [ lo...
db.users.aggregate([{$match:{age:{$gt:20}}},{$group:{_id:null,count:{$sum:1}}}]) 1. 2. 3. 4. 这将返回一个包含用户数量的文档。 旅行图 以下是MongoDB集合查询的旅行图,描述了查询操作的过程: journey title MongoDB集合查询 section 查询数据 查询条件 --> 查询结果 查询结果 --> 投影...
{ $pullAll: { <field1>: [ <value1>, <value2> ... ], ... } } $pull 符合条件的值将被删除 { $pull: { <field1>: <value|condition>, <field2>: <value|condition>, ... } } $pushAll 向数组中追加多个指定值 { $pushAll: { <field>: [ <value1>, <value2>, ... ] } }...
$exists #字段是否存在: {"name" :{$exists:true}} $gt #> $gte #>= $lt #< $lte #<= $in #in 交集: { "idlist":{$in:[1,2]} }或{"idlist" : [1,2]} $nin #not in $ne #!=,not null:{name: {$ne:null} } $not #!
db.yunfan_test.aggregate({$group:{_id:'$age',count_age:{$sum:1}}} );# 统计所有人平均年龄 db.yunfan_test.aggregate({$group:{_id:null,总人数:{$sum:1},avg_age:{$avg:"$age"},min_age:{$min:"$age"},max_age:{$max:"$age"}}} ...
db.xxxxx.aggregate({ "$indexStats":{}}) ; {"name" : "alxxxId_1_parentItxxxId_1_parentAlxxxId_1", "key" :{ "alxxxId" : 1, "parentItxxxId" : 1, "parentAlxxxId" : 1} ,"host" : "TENCENT64.site:7014", "accesses" : {"ops" : NumberLong 11236765 ,"since" : ISODate "...
] db.aggregate([pipeline], {options}) - performs a collectionless aggregation on this database; returns a cursor db.auth(username, password) db.cloneDatabase(fromhost) db.commandHelp(name) returns the help for the command db.copyDatabase(fromdb, todb, fromhost) db.createCollection(name, {...
The following aggregation uses the$regexMatchto check if thecommentfield contains an email address with@mongodb.comand categorize the feedback asEmployeeorExternal. db.feedback.aggregate([ {$addFields:{ "category":{$cond:{if:{$regexMatch:{input:"$comment",regex:/[a-z0-9_.+-]+@mongodb.co...
> db.help() DB methods: db.adminCommand(nameOrDocument) - switches to 'admin' db, and runs command [just calls db.runCommand(...)] db.aggregate([pipeline], {options}) - performs a collectionless aggregation on this database; returns a cursor db.auth(username, password) db.cloneDatabase...