Question

I need to index a collection by two fields (unique index), say field1 and field2. What’s better approach in terms of performance:

1. Create a regular two-column index

   -or –

2. Combine those two fields in a single document field {field1 : value, field2 : value2} and index that field?

Note: I will always be querying by those two fields together.

Answer 1

You can keep the columns separate and create a single index that will increase performance when querying both fields together.

db.things.ensureIndex({field1:1, field2:1});

http://www.mongodb.org/display/DOCS/Indexes#Indexes-CompoundKeysIndexes

Having the columns in the same column provides no performance increases, because you must index them the same way:

db.things.ensureIndex({fields.field1:1, fields.field2:1});

http://www.mongodb.org/display/DOCS/Indexes#Indexes-EmbeddedKeys

Or you can index the entire document

db.things.ensureIndex({fields: 1});

http://www.mongodb.org/display/DOCS/Indexes#Indexes-DocumentsasKeys

There could be a possible performance increase, but doubtfully very much. Use the test database, create test data and benchmark some tests to figure it out. We would love to hear your results.