首先看一下操作过程：
mongos> db.find({ "user" : "jhon"}).sort({"name" :
1}).limit(100).explain()
{
"cursor" : "BtreeCursor user_1",
"nscanned" : 10100,
"nscannedObjects" : 10100,
"n" : 100,
"scanAndOrder" : true,
"millis" : 116,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"user" : [
[
"jhon",
"jhon"
]
]
}
}
Second, I do $or query with sort():
mongos> db.find({ "$or" : [ { "user" : "jhon"} , { "owner" :
"jhon"}]}).sort({"name" : 1}).limit(100).explain()
{
"cursor" : "BtreeCursor name_1",
"nscanned" : 1010090,
"nscannedObjects" : 1010090,
"n" : 100,
"millis" : 3800,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"name" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
}
}
Last, I do $or query without sort():
mongos> db.find({ "$or" : [ { "user" : "jhon"} , { "owner" :
"jhon"}]}).limit(100).explain()
{
"cursor" : "BtreeCursor user_1",
"nscanned" : 100,
"nscannedObjects" : 100,
"n" : 100,
"millis" : 0,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"user" : [
[
"jhon",
"jhon"
]
]
}
}
可以看出：
第一次查询中， 单独使用sort()时性能很好。
第二次查询中，联合使用了$or和sort()时，性能很差。
第三次查询中，单独使用$or,性能很好。
后来在官方论坛中提问，得知有一个bug：https://jira.mongodb.org/browse/SERVER-1205
将来会修改，今天先记录一下。
论坛回复:
I believe the issue you are running into is expressed in this JIRA
ticket: https://jira.mongodb.org/browse/SERVER-1205
I believe the query optimizer is choosing to use the name index and
walk it backwards. As it goes through the index it compares the user
and owner attributes to your parameters and collects them in sorted
order. As a result the nscanned objects is much larger than in the
other two cases.
I would vote up the issue to prioritize it.
-Tyler
注：如果查询中使用了$or，并且查询元素有索引的话，那么也会使用该元素的索引的。换句话说，如果查询中只有$or时（无sort时），不会出现这里的问题。另外在使用$and和$or的组合时也发现了类似的问题。