The $or operator performs a logical OR operation on an array of two or more <expressions> and selects the documents that satisfy at least one of the <expressions>. The $or has the following syntax:

{ $or: [ { <expression1> }, { <expression2> }, ... , { <expressionN> } ] }

Consider the following example:

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

This query will select all documents in the inventory collection where either the quantity field value is less than 20 or the price field value equals 10.

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.

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 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.

When executing $or queries with a sort(), MongoDB can now use indexes that support the $or clauses. Previous versions did not use the indexes.

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] } } )

You may nest $or operations.

See also:
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