Multikey Indexes

Overview

Multikey indexes are indexes that improve performance for queries on array-valued fields. You can define a multikey index by using the same syntax as a single field or compound index.

Create a Multikey Index

The following example creates a multikey index on the cast field:

bson_error_t error;
bson_t *keys = BCON_NEW ("cast", BCON_INT32 (1));
mongoc_index_model_t *index_model = mongoc_index_model_new (keys, NULL);
if (mongoc_collection_create_indexes_with_opts (collection, &index_model, 1, NULL, NULL, &error)) {
    printf ("Successfully created index\n");
} else {
    fprintf (stderr, "Failed to create index: %s", error.message);
}
bson_destroy (keys);
mongoc_index_model_destroy (index_model);

The following example performs a query that uses the index created in the preceding code sample:

const bson_t *doc;
bson_t *filter = BCON_NEW ("cast", BCON_UTF8 ("Viola Davis"));
mongoc_cursor_t *results =
    mongoc_collection_find_with_opts (collection, filter, NULL, NULL);
while (mongoc_cursor_next (results, &doc)) {
    char *str = bson_as_canonical_extended_json (doc, NULL);
    printf ("%s\n", str);
    bson_free (str);
}
mongoc_cursor_destroy (results);
bson_destroy (filter);

{ "_id" : ..., "cast" : [ "Kelsey Grammer", "Cary Elwes", "Viola Davis", "John C. McGinley" ], "title" : "The Pentagon Wars", ... }
{ "_id" : ..., "cast" : [ "George Clooney", "Natascha McElhone", "Viola Davis", "Jeremy Davies" ], "title" : "Solaris", ... }
{ "_id" : ..., "cast" : [ "Meryl Streep", "Philip Seymour Hoffman", "Amy Adams", "Viola Davis" ], "title" : "Doubt", ... }
{ "_id" : ..., "cast" : [ "Hugh Jackman", "Jake Gyllenhaal", "Viola Davis", "Maria Bello" ], "title" : "Prisoners", ... }
{ "_id" : ..., "cast" : [ "Emma Stone", "Viola Davis", "Bryce Dallas Howard", "Octavia Spencer" ], "title" : "The Help", ... }
...

Multikey indexes behave differently from other indexes in terms of query coverage, index bound computation, and sort behavior. To learn more about multikey indexes, including a discussion of their behavior and limitations, see the Multikey Indexes guide in the MongoDB Server manual.