Mongo index doesn't make query faster

Question

Solved. Just change db.test.ensureIndex(['name']) => to db.test.ensureIndex({'name':1});

I was testing MongoDB today. I have test collection with 100 000 documents. Each document has such structure {_id:123123, name: 'foo123123'}

Test code written in PHP

set_time_limit(0);
ini_set('display_errors', 1);

$mongo = new Mongo("mongodb://127.0.0.1:27017");
$db = $mongo->test;
$collection = $db->test;

$start = microtime(true);

for($i=0; $i<10000; $i++){
    $obj = $collection->findOne(array('name'=>'foo'.$i));
}

$end = microtime(true);
var_dump($end-$start);

First test without index on name field is 21 seconds Than I add index

db.test.ensureIndex(['name']);

db.test.getIndexes(); [ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "test.test", "name" : "id" }, { "v" : 1, "key" : [ "name" ], "ns" : "test.test", "name" : "0_" } ]

And repeat test. And got 21 seonds again. Why mongo doesn't use index in my case?

Bryan Migliorisi · Accepted Answer · 2011-11-09 19:43:07Z

2

You should use the explain method to view the execution plan and find out whether or not it is using the index correctly.

http://www.mongodb.org/display/DOCS/Explain

I also think that you want to update your index creation command to this:

db.things.ensureIndex({"name": 1})

It isn't an array, but rather a key-value pair where the key is the property you want to index and the value is 1 or -1 depending on whether you want it to be indexed ascending or descending.

Check the docs for indexes: http://www.mongodb.org/display/DOCS/Indexes#Indexes-CreationOptions

edited Nov 9, 2011 at 19:43

answered Nov 9, 2011 at 19:27

Bryan Migliorisi

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7 Comments

zim32 Over a year ago

For my query it is { "cursor" : "BasicCursor", "nscanned" : 100000, "nscannedObjects" : 100000, "n" : 1, "millis" : 68, "nYields" : 0, "nChunkSkips" : 0, "isMultiKey" : false, "indexOnly" : false, "indexBounds" : { } }

Bryan Migliorisi Over a year ago

So then youre correct that it is not using an index - if it were, it would say that the cursor were a B-Tree Index rather than a basic cursor.

Bryan Migliorisi Over a year ago

I've updated my answer with more information about indexes - please have a look

zim32 Over a year ago

Don't you know why it happens?

zim32 Over a year ago

Thanks! You are right. I change my command to db.things.ensureIndex({"name": 1}) and now it is using index

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