-
Type:
Improvement
-
Resolution: Done
-
Priority:
Major - P3
-
Affects Version/s: None
-
Component/s: Index Maintenance, Querying
-
Fully Compatible
> c.ensureIndex({a:1})
> c.find({a:{$gt:5}})
> c.find({a:{$gt:5}}).explain()
{
"cursor" : "BtreeCursor a_1",
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 7,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"a" : [
[
5,
1.7976931348623157e+308
]
]
}
}
> c.find({a:{$gte:5}}).explain()
{
"cursor" : "BtreeCursor a_1",
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 0,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"a" : [
[
5,
1.7976931348623157e+308
]
]
}
}
With an exclusive bound our btree scanning implementation can avoid scanning every key with a=5, while with an inclusive bound we must scan every key with a=5. However, there is no indication of this from the explain output. I usually mention that this is the case when doing new hire training, and recently it was raised specifically as a usability issue.