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  1. PO
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    <p>That was funny i cannot decide to say it is a bug or not it is up to you:</p> <p>There are two available syntax: <a href="http://docs.mongodb.org/manual/reference/operator/query/">http://docs.mongodb.org/manual/reference/operator/query/</a></p> <p>When you using: </p> <pre><code>db.collection.find( { age : 25 } ) </code></pre> <p>also will </p> <pre><code>db.collection.find( { age : 25 } ).explain() db.collection.find( { age : 25 } ).hint(someindex) </code></pre> <p>work fine.</p> <p>When you using your solution (the other syntax):</p> <pre><code>db.collection.find( { $query: { age : 25 } } ) </code></pre> <p>the output of </p> <pre><code>db.sampleCollection.find({$query:{"stringField":"Random string0"}}).explain() </code></pre> <p>Will show like the query not using the index</p> <p>if you also use .hint for the index it will omit the result. :) (That is i do not really understand)</p> <p>Fortunately there is another syntax for these operations too: you can use:</p> <pre><code>db.sampleCollection.find({$query:{"stringField":"Random string0"}, $explain:1}) </code></pre> <p>it will have the right output and showed for me the usage of the index. Also there is similar syntax for $hint.</p> <p>You can check the documentation here: <a href="http://docs.mongodb.org/manual/reference/meta-query-operators/">http://docs.mongodb.org/manual/reference/meta-query-operators/</a></p> <p>I found this really interesting so i turned on the profiler: </p> <p>i made a test collection (queryTst) with around 250k docs each with only _id and an age field in the structure with an index on age.</p> <p>For this query:</p> <pre><code>db.queryTst.find({$query:{"age":16},$explain:1}) </code></pre> <p>i got:</p> <pre><code>{ "cursor" : "BtreeCursor age_1", "isMultiKey" : false, "n" : 2, "nscannedObjects" : 2, "nscanned" : 2, "nscannedObjectsAllPlans" : 2, "nscannedAllPlans" : 2, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 0, "indexBounds" : { "age" : [ [ 16, 16 ] ] }, "allPlans" : [ { "cursor" : "BtreeCursor age_1", "n" : 2, "nscannedObjects" : 2, "nscanned" : 2, "indexBounds" : { "age" : [ [ 16, 16 ] ] } } ], "oldPlan" : { "cursor" : "BtreeCursor age_1", "indexBounds" : { "age" : [ [ 16, 16 ] ] } }, "server" : "" } </code></pre> <p>for this:</p> <pre><code> db.queryTst.find({$query:{"age":16},$explain:1}).explain() </code></pre> <p>i got:</p> <pre><code>"cursor" : "BasicCursor", "isMultiKey" : false, "n" : 0, "nscannedObjects" : 250011, "nscanned" : 250011, "nscannedObjectsAllPlans" : 250011, "nscannedAllPlans" : 250011, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 103, "indexBounds" : { }, </code></pre> <p>in the profiler log: for the first</p> <pre><code>{ "ts" : ISODate("2013-01-30T20:35:40.526Z"), "op" : "query", "ns" : "test.queryTst", "query" : { "$query" : { "age" : 16 }, "$explain" : 1 }, "ntoreturn" : 0, "ntoskip" : 0, "nscanned" : 2, "keyUpdates" : 0, "numYield" : 0, "lockStats" : { "timeLockedMicros" : { "r" : NumberLong(368), "w" : NumberLong(0) }, "timeAcquiringMicros" : { "r" : NumberLong(8), "w" : NumberLong(5) } }, "nreturned" : 1, "responseLength" : 567, "millis" : 0, "client" : "127.0.0.1", "user" : "" } </code></pre> <p>for the second:</p> <pre><code>{ "ts" : ISODate("2013-01-30T20:35:47.715Z"), "op" : "query", "ns" : "test.queryTst", "query" : { "query" : { "$query" : { "age" : 16 }, "$explain" : 1 }, "$explain" : true }, "ntoreturn" : 0, "ntoskip" : 0, "nscanned" : 250011, "keyUpdates" : 0, "numYield" : 0, "lockStats" : { "timeLockedMicros" : { "r" : NumberLong(104092), "w" : NumberLong(0) }, "timeAcquiringMicros" : { "r" : NumberLong(13), "w" : NumberLong(5) } }, "nreturned" : 1, "responseLength" : 373, "millis" : 104, "client" : "127.0.0.1", "user" : "" } </code></pre> <p>which somehow means to me that is the explain() cause the table scan in the mixed syntax.</p>
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    1. COYes it is a bug in the explain but it doesn't answer why.
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    2. COThere are two syntaxes and did not made for using mixed (both made to have same functionality? As i could see the docs , yes), they are not seem to be cross compatible.
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    3. COThe two are analogous of each other, as it says in the docs. But as to them being cross compatible; I am still unsure what you mean there.
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