【MongoDB学习笔记九】MongoDB索引

索引

• 可以在任意列上建立索引
• 索引的构造和使用与传统关系型数据库几乎一样,适用于Oracle的索引优化技巧也适用于
  Mongodb
• 使用索引可以加快查询,但同时会降低修改,插入等的性能
• 内嵌文档照样可以建立使用索引
  

测试数据

var p1 = {
"name":"Jack",
"age":34,
"nickname":"xiaoqiang",
friends:[
  {"name":"Cloe","age":"28"},	
  {"name":"Audrey","age":"31"},
]
}


var p2 = {
"name":"Cloe",
"age":28,
friends:[
  {"name":"Jack","age":"34"},	
  {"name":"Mike","age":"31"},
]
}

var p3 = {
"name":"Audrey",
"age":31,
friends:[
  {"name":"Jack","age":"34"},	
  {"name":"Hammer","age":"61","relation":"parent"},
]
}

var p = [p1,p2, p3];
db.people.drop();
db.people.insert(p);

建立索引

当查询列中包含name属性时，索引将其作用

db.people.ensureIndex({"name" : 1}) //1表示正向索引

建立复合索引

复合索引的起作用是当name和age属性相邻,复合索引起作用

db.people.ensureIndex({"name" : 1，"age" : -1}) //1表示正向索引,-1表示反向索引

查询计划查看索引是否作用

查询时，哪个索引起作用与find中定义的查询对象(JSON)的书写顺序无关，这是合理的，因为JSON对象本身是键值对，无序的键值集合

1. db.people.find().explain()

> db.people.find().explain();
{
	"cursor" : "BasicCursor", //因为查询属性不包含定义索引的属性，因此索引没有起作用
	"isMultiKey" : false,
	"n" : 3,
	"nscannedObjects" : 3,
	"nscanned" : 3,
	"nscannedObjectsAllPlans" : 3,
	"nscannedAllPlans" : 3,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 11,
	"indexBounds" : {
		
	},
	"server" : "tom-Inspiron-3521:27017"
}

2. db.people.find({"name":"Jack"});

> db.people.find({"name":"Jack"}).explain();
{
	"cursor" : "BtreeCursor name_1", //使用B数游标 name_1就是创建于name属性上的索引
	"isMultiKey" : false,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 1,
	"nscannedAllPlans" : 1,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 10,
	"indexBounds" : {
		"name" : [
			[
				"Jack",
				"Jack"
			]
		]
	},
	"server" : "tom-Inspiron-3521:27017"
}

 3. db.people.find({"name":"Jack","age":34});

> db.people.find({"name":"Jack","age":34}).explain();
{
	"cursor" : "BtreeCursor name_1_age_-1", //使用了B树索引，定义于name和age上的索引
	"isMultiKey" : false,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 1,
	"nscannedAllPlans" : 1,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"indexBounds" : {
		"name" : [
			[
				"Jack",
				"Jack"
			]
		],
		"age" : [
			[
				34,
				34
			]
		]
	},
	"server" : "tom-Inspiron-3521:27017"
}

  4. db.people.find({"age":34, "name":"Jack"});

}
> db.people.find({"age":34, "name":"Jack"}).explain();
{
	"cursor" : "BtreeCursor name_1_age_-1",//age和name的次序颠倒不影响索引的使用
	"isMultiKey" : false,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 1,
	"nscannedAllPlans" : 1,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"indexBounds" : {
		"name" : [
			[
				"Jack",
				"Jack"
			]
		],
		"age" : [
			[
				34,
				34
			]
		]
	},
	"server" : "tom-Inspiron-3521:27017"
}

 5. db.people.find({"age":34, "nickname":"xiaoqiang", "name":"Jack"});

db.people.find({"age":34, "nickname":"xiaoqiang", "name":"Jack"}).explain();
{
	"cursor" : "BtreeCursor name_1", //因为name索引列定义了，因此使用name索引
	"isMultiKey" : false,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 3,
	"nscannedAllPlans" : 3,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 17,
	"indexBounds" : {
		"name" : [
			[
				"Jack",
				"Jack"
			]
		]
	},
	"server" : "tom-Inspiron-3521:27017"
}

 6. db.people.find({ "name":"Jack","age":34, "nickname":"xiaoqiang"});

 db.people.find({ "name":"Jack","age":34, "nickname":"xiaoqiang"}).explain(); //name,age，nickname数序
{
	"cursor" : "BtreeCursor name_1", //name属性定义的索引起作用
	"isMultiKey" : false,
	"n" : 1,
	"nscannedObjects" : 1,
	"nscanned" : 1,
	"nscannedObjectsAllPlans" : 3,
	"nscannedAllPlans" : 3,
	"scanAndOrder" : false,
	"indexOnly" : false,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"millis" : 0,
	"indexBounds" : {
		"name" : [
			[
				"Jack",
				"Jack"
			]
		]
	},
	"server" : "tom-Inspiron-3521:27017"
}

查询计划结果说明

• cursor 用于指明这个查询，是否使用了索引，哪个属性使用索引
• nsScanned 用于指明本次查询，数据库扫描的文档数一共多少。最佳情况是这个数字应该尽可能的接近查询结果返回的文档数
• n 查询结果返回的文档数
• millis 执行查询耗费的毫秒数

索引名

上面的例子中，索引的名字都是MongoDB自动创建的，如何创建索引名呢？

唯一索引

为内嵌文档的属性设置索引

查看索引

索引建好后，如何察看当前的数据库有哪些索引呢？可见，默认情况下，每个集合的_id属性都已经是正向索引，这是MongoDB内置提供的

> db.system.indexes.find();
{ "v" : 1, "name" : "_id_", "key" : { "_id" : 1 }, "ns" : "foobar.persons" }
{ "v" : 1, "name" : "_id_", "key" : { "_id" : 1 }, "ns" : "foobar.db-text" }
{ "v" : 1, "name" : "_id_", "key" : { "_id" : 1 }, "ns" : "foobar.addresses" }
{ "v" : 1, "name" : "_id_", "key" : { "_id" : 1 }, "ns" : "foobar.c" }
{ "v" : 1, "name" : "_id_", "key" : { "_id" : 1 }, "ns" : "foobar.people" }
{ "v" : 1, "name" : "name_1", "key" : { "name" : 1 }, "ns" : "foobar.people" }
{ "v" : 1, "name" : "name_1_age_-1", "key" : { "name" : 1, "age" : -1 }, "ns" : "foobar.people" }

 

 删除索引

> db.runCommand({"dropIndexes":"db.people","index":"name_1"}); //db.people不工作
{ "ok" : 0, "errmsg" : "ns not found" }
> db.runCommand({"dropIndexes":"foobar.people","index":"name_1"}); //foobar.people不工作
{ "ok" : 0, "errmsg" : "ns not found" }
> db.runCommand({"dropIndexes":"people","index":"name_1"}); //使用people，即指定了集合的名称即可
{ "nIndexesWas" : 3, "ok" : 1 }