Performing Queries: Overview

• How do you establish query criteria?

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  • Criteria consists of one or more fields
  • Use the minimum number of fields to identify the document
  • Demo: customers collection

• What query operators are available?

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  • Query operators modify the query
  • Comparison: $gt, $gte, $in, $lt, $lte, $ne, $nin
  • Logical: $and, $or, $not, $nor
  • Element: $exists, $type
  • Evaluation: $mod, $regex, $where
  • Geospatial: $geoWithin, $geoIntersects, $near, $nearSphere
  • Array: $all, $elemMatch, $size

• What are projections?

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  • find( { query }, { projection } )
  • Allows you to specify which fields are returned
  • In the projection document, 1 = include, 0 = exclude
  • find( {query},{name:1,_id:0})

• What projection operators are available?

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  • $, $elemMatch, $slice

• What about modifiers?

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  • Included in the query:
  • $comment, $explain, $hint, $maxScan, $max, $min
  • $orderBy, $returnKey, $showDiskLoc, $snapshot, $query, $natural
  • Outside the query:
  • explain(), hint(), limit(), max(), min(), sort(), snapshot()

• How can you optimize queries?

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  • Create an index to cover fields used in query
  • db.collection.ensureIndex({field:1})
  • 1 = ascending, -1 = descending
  • Use projections and limit() to limit fields and documents returned
  • Use explain() to analyze performance
  • Use hint() to force MongoDB to use a particular index