$or

$or Clauses and Indexes

When evaluating the clauses in the $or expression, MongoDB either performs a collection scan or, if all the clauses are supported by indexes, MongoDB performs index scans. That is, for MongoDB to use indexes to evaluate an $or expression, all the clauses in the $or expression must be supported by indexes. Otherwise, MongoDB will perform a collection scan.

When using indexes with $or queries, each clause of an $or can use its own index. Consider the following query:

db.inventory.find( { $or: [ { quantity: { $lt: 20 } }, { price: 10 } ] } )

To support this query, rather than a compound index, you would create one index on quantity and another index on price:

db.inventory.createIndex( { quantity: 1 } )
db.inventory.createIndex( { price: 1 } )

MongoDB can use all but the geoHaystack index to support $or clauses.

$or and text Queries

If $or includes a $text query, all clauses in the $or array must be supported by an index. This is because a $text query must use an index, and $or can only use indexes if all its clauses are supported by indexes. If the $text query cannot use an index, the query will return an error.

$or and GeoSpatial Queries

$or supports geospatial clauses with the following exception for the near clause (near clause includes $nearSphere and $near). $or cannot contain a near clause with any other clause.

$or and Sort Operations

When executing $or queries with a sort(), MongoDB can use indexes that support the $or clauses.

$or versus $in

When using $or with <expressions> that are equality checks for the value of the same field, use the $in operator instead of the $or operator.

For example, to select all documents in the inventory collection where the quantity field value equals either 20 or 50, use the $in operator:

db.inventory.find ( { quantity: { $in: [20, 50] } } )

Nested $or Clauses

You may nest $or operations.

Error Handling

To allow the query engine to optimize queries, $or handles errors as follows:

• If any expression supplied to $or would cause an error when evaluated alone, the $or containing the expression may cause an error but an error is not guaranteed.

• An expression supplied after the first expression supplied to $or may cause an error even if the first expression evaluates to true.

For example, the following query always produces an error if $x is 0:

db.example.find( {
   $expr: { $eq: [ { $divide: [ 1, "$x" ] }, 3 ] }
} )

The following query, which contains multiple expressions supplied to $or, may produce an error if there is any document where $x is 0:

db.example.find( {
   $or: [
      { x: { $eq: 0 } },
      { $expr: { $eq: [ { $divide: [ 1, "$x" ] }, 3 ] } }
   ]
} )