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https://issues.apache.org/jira/browse/OAK-2007?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14079298#comment-14079298
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Amit Jain commented on OAK-2007:
--------------------------------
With the index hint the query uses the index on the {{_id}} and without which
it does not.
{code}
> db.blobs.find({lastMod : {$lte :1406726400279}}, {_id:1}).explain()
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 14,
"nscannedObjects" : 3236,
"nscanned" : 3236,
"nscannedObjectsAllPlans" : 3236,
"nscannedAllPlans" : 3236,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 25,
"nChunkSkips" : 0,
"millis" : 18,
"server" : "amjain-W7-1:27017",
"filterSet" : false
}
> db.blobs.find({lastMod : {$lte :1406726400279}},
> {_id:1}).hint({_id:1}).explain()
{
"cursor" : "BtreeCursor _id_",
"isMultiKey" : false,
"n" : 14,
"nscannedObjects" : 3399,
"nscanned" : 3399,
"nscannedObjectsAllPlans" : 3399,
"nscannedAllPlans" : 3399,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 26,
"nChunkSkips" : 0,
"millis" : 38,
"indexBounds" : {
"_id" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
},
"server" : "amjain-W7-1:27017",
"filterSet" : false
}
{code}
I think the problem is that with the {{lastMod}} condition, the {{_id}} index
does not cover the query any more and I am not sure how much an index on
{{lastMod}} would help, because it would still not cover the query. It might
be better to have a compound index of the form {code}{lastMod:1, id:1}{code}
I am not sure of the impact on performance if any on insert performance though.
> MongoBlobStore improvements
> ---------------------------
>
> Key: OAK-2007
> URL: https://issues.apache.org/jira/browse/OAK-2007
> Project: Jackrabbit Oak
> Issue Type: Improvement
> Components: blob, mongomk
> Affects Versions: 1.0.2
> Reporter: Przemo Pakulski
> Attachments: OAK-2007.patch
>
>
> To collect all chunk identifiers (getAllChunkIds) hint is used to force "_id"
> index usage.
> This index doesn't help because query is using "lastMod" field. It also
> doesn't allow to leverage custom indexes if created. In result queries are
> pretty slow.
> Additionally consider to create index on "lastMod" field to speed-up all
> queries using this criteria.
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