我有一个具有相对较大索引的集合(但小于可用内存),并查看该集合上 find 的性能以及 htop 给出的系统中的可用内存量,似乎 mongo 没有在内存中存储完整索引。有没有办法强制 mongo 将这个特定索引存储在内存中?
查询示例:
> db.barrels.find({"tags":{"$all": ["avi"]}}).explain()
{
"cursor" : "BtreeCursor tags_1",
"nscanned" : 300393,
"nscannedObjects" : 300393,
"n" : 300393,
"millis" : 55299,
"indexBounds" : {
"tags" : [
[
"avi",
"avi"
]
]
}
}
并非所有对象都带有“avi”标签:
> db.barrels.find().explain()
{
"cursor" : "BasicCursor",
"nscanned" : 823299,
"nscannedObjects" : 823299,
"n" : 823299,
"millis" : 46270,
"indexBounds" : {
}
}
没有“$all”:
db.barrels.find({"tags": ["avi"]}).explain()
{
"cursor" : "BtreeCursor tags_1 multi",
"nscanned" : 300393,
"nscannedObjects" : 300393,
"n" : 0,
"millis" : 43440,
"indexBounds" : {
"tags" : [
[
"avi",
"avi"
],
[
[
"avi"
],
[
"avi"
]
]
]
}
}
当我搜索两个或多个标签时也会发生这种情况(它会扫描每个项目,就好像没有索引一样):
> db.barrels.find({"tags":{"$all": ["avi","mp3"]}}).explain()
{
"cursor" : "BtreeCursor tags_1",
"nscanned" : 300393,
"nscannedObjects" : 300393,
"n" : 6427,
"millis" : 53774,
"indexBounds" : {
"tags" : [
[
"avi",
"avi"
]
]
}
}