MongoDB, performance of query by regular expression on indexed fields

by

Actually according to the documentation,

If an index exists for the field, then MongoDB matches the regular
expression against the values in the index, which can be faster than a
collection scan. Further optimization can occur if the regular
expression is a “prefix expression”, which means that all potential
matches start with the same string. This allows MongoDB to construct a
“range” from that prefix and only match against those values from the
index that fall within that range.

http://docs.mongodb.org/manual/reference/operator/query/regex/#index-use

In other words:

For /Jon Skeet/ regex ,mongo will full scan the keys in the index then will fetch the matched documents, which can be faster than collection scan.

For /^Jon Skeet/ regex ,mongo will scan only the range that start with the regex in the index, which will be faster.

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