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I don't have an idea what "state to the left" in the bolded sentence below means. 
Does anyone kindly help me grasp the meaning?
(I know maybe it's not an english forum but an IT forum I should go to find the answer. But at least, I think, you can tell me if the sentence is not that clear.)
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MongoDB supports compound indexes. If you commonly query on two fields.say, state and city.then you should create a compound index containing both fields. You wouldn’t want to create two indexes (one on each), as only one would be used and either one would result in a fair amount of scanning on disk. In creating and using compound indexes, order matters. In this example, you would want to make sure that you create the index with state to the left. The syntax for creating a compound index is by simply passing an array to the ensure index method. The following example would be useful when searching for blog posts in a given time range where a specific author commented: 

$blogpost->ensureIndex(array(
"ts" => -1,
 "comments.author" => 1 
)); 
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It seems to mean literally that the name of the US state is positioned to the left of the city name, for example:

Idaho, Boise.
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Your comment perfectly makes sense with the other sentences.

I was confused and thought 'this example' in the colored sentence and 'The following example' in the last sentence direct the same thing.

Thanks!
I appreciate your help Emotion: wink
Right. It is not very well written. "This example" is the "city, state" search index. I guess the state comes first because there are fewer of them, and the search steps left to right through the fields. The code sample at the bottom is for a different two-field array involving time and name and illustrates how to create one.