MongoDB使用游标来处理find查询到的结果文档集;
游标的使用实例:
> var cursor=db.post.find({"name.firstname":"joe"}) > cursor { "_id" : ObjectId("54ace1394ba07ed75df68f90"), "name" : { "firstname" : "joe", "lastname" : "schome" }, "age" : 28 } { "_id" : ObjectId("54ace753eab1d0ba4abb48ab"), "name" : { "firstname" : "joe", "lastname" : "schome" }, "age" : 30 } { "_id" : ObjectId("54ace777eab1d0ba4abb48ac"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 10 }, "age" : 30 } { "_id" : ObjectId("54ace781eab1d0ba4abb48ad"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 6 }, "age" : 30 } { "_id" : ObjectId("54ace785eab1d0ba4abb48ae"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 5 }, "age" : 30 } { "_id" : ObjectId("54ace789eab1d0ba4abb48af"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 7 }, "age" : 30 } >
使用limit来返回前两个记录:
> cursor.limit(2) { "_id" : ObjectId("54ace1394ba07ed75df68f90"), "name" : { "firstname" : "joe", "lastname" : "schome" }, "age" : 28 } { "_id" : ObjectId("54ace753eab1d0ba4abb48ab"), "name" : { "firstname" : "joe", "lastname" : "schome" }, "age" : 30 } >
或者使用skip来跳过前3个记录:
> cursor.skip(3) { "_id" : ObjectId("54ace781eab1d0ba4abb48ad"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 6 }, "age" : 30 } { "_id" : ObjectId("54ace785eab1d0ba4abb48ae"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 5 }, "age" : 30 } {"_id" : ObjectId("54ace789eab1d0ba4abb48af"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 7 }, "age" : 30 } >
或者使用sort指定键值排序:
> cursor.sort({"age":1}) { "_id" : ObjectId("54ace1394ba07ed75df68f90"), "name" : { "firstname" : "joe", "lastname" : "schome" }, "age" : 28 } { "_id" : ObjectId("54ace753eab1d0ba4abb48ab"), "name" : { "firstname" : "joe", "lastname" : "schome" }, "age" : 30 } { "_id" : ObjectId("54ace777eab1d0ba4abb48ac"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 10 }, "age" : 30 } { "_id" : ObjectId("54ace781eab1d0ba4abb48ad"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 6 }, "age" : 30 } { "_id" : ObjectId("54ace785eab1d0ba4abb48ae"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 5 }, "age" : 30 } { "_id" : ObjectId("54ace789eab1d0ba4abb48af"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 7 }, "age" : 30 }
综合使用sort/limit/skip,当然可以调整顺序得到不同的结果:
> cursor.sort({"name.comment":1}).skip(2).limit(1) { "_id" : ObjectId("54ace785eab1d0ba4abb48ae"), "name" : { "firstname" : "joe", "lastname" : "schome", "comment" : 5 }, "age" : 30 } >
游标生命周期:
游标会消耗内存和其他资源,游标使用完后会释放资源以供数据库它用;游标会在以下情况终止,
(1)游标完成匹配结果的迭代后,它会清楚自身;
(2)客户端的游标已经不再作用域内,驱动程序回向服务器发送一条特别的消息,让其销毁;
(3)游标在十分钟内没有使用,游标自动销毁;
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