Indexing and Performance Considerations

• How does an index affect performance?

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  • Indexes will greatly enhance database queries

• How do you create an index?

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  • Create an index on fields usually involved in queries
  • db.collection.ensureIndex({field(s)})

• How can you reduce MongoDB file sizes?

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  • MongoDB pre-allocates space to increase speed
  • Change the smallfiles setting in mongod.conf

• How can you reduce the overhead for each document?

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  • By default _id is a 12 byte ObjectId
  • Explicitly set _id to a smaller value
  • Keep field names short
  • Use embedded documents as much as possible

• What about using GridFS for large files?

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  • Store large files as GridFS objects
  • Works well for files larger than 16MB in size
  • Use the mongofiles command from the command line
  • From an application, usage varies
  • PHP uses the MongoGridFS class
  • More info: docs.mongodb.org/manual/core/gridfs/
    docs.mongodb.org/manual/reference/program/mongofiles/

• What affect does replication have on performance?

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  • Read concerns allow you to influence which replica set responds
  • Read concerns can potentially improve performance

• What affect do "write concerns" have on performance?

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  • Strengthening write concerns will improve reliability but reduce performance