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Text Indexes¶
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Changed in version 3.2.
Starting in MongoDB 3.2, MongoDB introduces a version 3 of the text index. Key features of the new version of the index are:
- Improved case insensitivity
- Diacritic insensitivity
- Additional delimiters for tokenization
Starting in MongoDB 3.2, version 3 is the default version for new text indexes.
Overview¶
MongoDB provides text indexes to support query operations that perform a text search of string content. text indexes can include any field whose value is a string or an array of string elements.
Create Text Index¶
Important
A collection can have at most one text index.
To create a text index, use the db.collection.createIndex() method. To index a field that contains a string or an array of string elements, include the field and specify the string literal "text" in the index document, as in the following example:
db.reviews.createIndex( { comments: "text" } )
You can index multiple fields for the text index. The following example creates a text index on the fields subject and comments:
db.reviews.createIndex(
{
subject: "text",
comments: "text"
}
)
A compound index can include text index keys in combination with ascending/descending index keys. For more information, see Compound Index.
Specify Weights¶
For a text index, the weight of an indexed field denotes the significance of the field relative to the other indexed fields in terms of the text search score.
For each indexed field in the document, MongoDB multiplies the number of matches by the weight and sums the results. Using this sum, MongoDB then calculates the score for the document. See $meta operator for details on returning and sorting by text scores.
The default weight is 1 for the indexed fields. To adjust the weights for the indexed fields, include the weights option in the db.collection.createIndex() method.
For more information using weights to control the results of a text search, see Control Search Results with Weights.
Wildcard Text Indexes¶
When creating a text index on multiple fields, you can also use the wildcard specifier ($**). With a wildcard text index, MongoDB indexes every field that contains string data for each document in the collection. The following example creates a text index using the wildcard specifier:
db.collection.createIndex( { "$**": "text" } )
This index allows for text search on all fields with string content. Such an index can be useful with highly unstructured data if it is unclear which fields to include in the text index or for ad-hoc querying.
Wildcard text indexes are text indexes on multiple fields. As such, you can assign weights to specific fields during index creation to control the ranking of the results. For more information using weights to control the results of a text search, see Control Search Results with Weights.
Wildcard text indexes, as with all text indexes, can be part of a compound indexes. For example, the following creates a compound index on the field a as well as the wildcard specifier:
db.collection.createIndex( { a: 1, "$**": "text" } )
As with all compound text indexes, since the a precedes the text index key, in order to perform a $text search with this index, the query predicate must include an equality match conditions a. For information on compound text indexes, see Compound Text Indexes.
Case Insensitivity¶
Changed in version 3.2.
The version 3 text index supports the common C, simple S, and for Turkish languages, the special T case foldings as specified in Unicode 8.0 Character Database Case Folding.
The case foldings expands the case insensitivity of the text index to include characters with diacritics, such as é and É, and characters from non-Latin alphabets, such as “И” and “и” in the Cyrillic alphabet.
Version 3 of the text index is also diacritic insensitive. As such, the index also does not distinguish between é, É, e, and E.
Previous versions of the text index are case insensitive for [A-z] only; i.e. case insensitive for non-diacritics Latin characters only . For all other characters, earlier versions of the text index treat them as distinct.
Diacritic Insensitivity¶
Changed in version 3.2.
With version 3, text index is diacritic insensitive. That is, the index does not distinguish between characters that contain diacritical marks and their non-marked counterpart, such as é, ê, and e. More specifically, the text index strips the characters categorized as diacritics in Unicode 8.0 Character Database Prop List.
Version 3 of the text index is also case insensitive to characters with diacritics. As such, the index also does not distinguish between é, É, e, and E.
Previous versions of the text index treat characters with diacritics as distinct.
Tokenization Delimiters¶
Changed in version 3.2.
For tokenization, version 3 text index uses the delimiters categorized under Dash, Hyphen, Pattern_Syntax, Quotation_Mark, Terminal_Punctuation, and White_Space in Unicode 8.0 Character Database Prop List.
For example, if given a string "Il a dit qu'il «était le meilleur joueur du monde»", the text index treats «, », and spaces as delimiters.
Previous versions of the index treat « as part of the term "«était" and » as part of the term "monde»".
Index Entries¶
text index tokenizes and stems the terms in the indexed fields for the index entries. text index stores one index entry for each unique stemmed term in each indexed field for each document in the collection. The index uses simple language-specific suffix stemming.
Supported Languages and Stop Words¶
MongoDB supports text search for various languages. text indexes drop language-specific stop words (e.g. in English, the, an, a, and, etc.) and use simple language-specific suffix stemming. For a list of the supported languages, see Text Search Languages.
If you specify a language value of "none", then the text index uses simple tokenization with no list of stop words and no stemming.
To specify a language for the text index, see Specify a Language for Text Index.
sparse Property¶
text indexes are sparse by default and ignore the sparse: true option. If a document lacks a text index field (or the field is null or an empty array), MongoDB does not add an entry for the document to the text index. For inserts, MongoDB inserts the document but does not add to the text index.
For a compound index that includes a text index key along with keys of other types, only the text index field determines whether the index references a document. The other keys do not determine whether the index references the documents or not.
Restrictions¶
One Text Index Per Collection¶
A collection can have at most one text index.
Text Index and Sort¶
Sort operations cannot obtain sort order from a text index, even from a compound text index; i.e. sort operations cannot use the ordering in the text index.
Compound Index¶
A compound index can include a text index key in combination with ascending/descending index keys. However, these compound indexes have the following restrictions:
- A compound text index cannot include any other special index types, such as multi-key or geospatial index fields.
- If the compound text index includes keys preceding the text index key, to perform a $text search, the query predicate must include equality match conditions on the preceding keys.
See also Text Index and Sort for additional limitations.
For an example of a compound text index, see Limit the Number of Entries Scanned.
Drop a Text Index¶
To drop a text index, pass the name of the index to the db.collection.dropIndex() method. To get the name of the index, run the db.collection.getIndexes() method.
For information on the default naming scheme for text indexes as well as overriding the default name, see Specify Name for text Index.
Storage Requirements and Performance Costs¶
text indexes have the following storage requirements and performance costs:
- text indexes can be large. They contain one index entry for each unique post-stemmed word in each indexed field for each document inserted.
- Building a text index is very similar to building a large multi-key index and will take longer than building a simple ordered (scalar) index on the same data.
- When building a large text index on an existing collection, ensure that you have a sufficiently high limit on open file descriptors. See the recommended settings.
- text indexes will impact insertion throughput because MongoDB must add an index entry for each unique post-stemmed word in each indexed field of each new source document.
- Additionally, text indexes do not store phrases or information about the proximity of words in the documents. As a result, phrase queries will run much more effectively when the entire collection fits in RAM.
Text Search Support¶
The text index supports $text query operations. For examples of text search, see the $text reference page. For examples of $text operations in aggregation pipelines, see Text Search in the Aggregation Pipeline.
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