Details
Description
New semantics for explain():
explain should re-run all plans for that query but NOT mutate the cache.
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In a full explain, the "allPlans" field can report the index bounds used when the query shape was cached. This is misleading, as it appears that some of the alternative plans used the wrong index bounds:
> t = db.t
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test.t
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> t.drop()
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true
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> t.ensureIndex({a: 1})
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WriteResult({ "nInserted" : 1 })
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> t.ensureIndex({a: 1, b: 1})
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WriteResult({ "nInserted" : 1 })
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> t.find({a: "foo"}).explain()
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{
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"cursor" : "BtreeCursor a_1",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 0,
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"nscanned" : 0,
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"nscannedObjectsAllPlans" : 0,
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"nscannedAllPlans" : 0,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 1,
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"indexBounds" : {
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"a" : [
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[
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"foo",
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"foo"
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]
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]
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},
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"server" : "localhost:27017",
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"filterSet" : false
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}
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> t.find({a: "bar"}).explain(true)
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{
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"cursor" : "BtreeCursor a_1",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 0,
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"nscanned" : 0,
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"nscannedObjectsAllPlans" : 0,
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"nscannedAllPlans" : 0,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nYields" : 0,
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"nChunkSkips" : 0,
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"millis" : 0,
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"indexBounds" : {
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"a" : [
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[
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"bar",
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"bar"
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]
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]
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},
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"allPlans" : [
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{
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"cursor" : "BtreeCursor a_1",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 0,
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"nscanned" : 0,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0,
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"indexBounds" : {
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"a" : [
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[
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"bar",
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"bar"
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]
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]
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}
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},
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{
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"cursor" : "BtreeCursor a_1_b_1",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 0,
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"nscanned" : 0,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0,
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"indexBounds" : {
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"a" : [
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[
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"foo",
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"foo"
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]
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],
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"b" : [
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[
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{
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"$minElement" : 1
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},
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{
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"$maxElement" : 1
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}
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]
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]
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}
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},
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{
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"cursor" : "BasicCursor",
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"isMultiKey" : false,
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"n" : 0,
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"nscannedObjects" : 0,
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"nscanned" : 0,
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"scanAndOrder" : false,
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"indexOnly" : false,
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"nChunkSkips" : 0
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}
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],
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"server" : "localhost:27017",
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"filterSet" : false,
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"stats" : {
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In the above example, the second plan reported in "allPlans" reports index bounds using "foo" rather than "bar".
Attachments
Issue Links
- is related to
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SERVER-12830 query does not select best index - uses wrong bounds in full explain
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- Closed
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- Closed
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