Index

public abstract @interface Index
implements Annotation

androidx.room.Index


Declares an index on an Entity. see: SQLite Index Documentation

Adding an index usually speeds up your SELECT queries but will slow down other queries like INSERT or UPDATE. You should be careful when adding indices to ensure that this additional cost is worth the gain.

There are 2 ways to define an index in an Entity. You can either set ColumnInfo.index() property to index individual fields or define composite indices via Entity.indices().

If an indexed field is embedded into another Entity via Embedded, it is NOT added as an index to the containing Entity. If you want to keep it indexed, you must re-declare it in the containing Entity.

Similarly, if an Entity extends another class, indices from the super classes are NOT inherited. You must re-declare them in the child Entity or set Entity.inheritSuperIndices() to true.

Summary

Nested classes

enum Index.Order

 

Public methods

String name()

Name of the index.

Order[] orders()

List of column sort orders in the Index.

boolean unique()

If set to true, this will be a unique index and any duplicates will be rejected.

String[] value()

List of column names in the Index.

Inherited methods

Public methods

name

public String name ()

Name of the index. If not set, Room will set it to the list of columns joined by '_' and prefixed by "index_${tableName}". So if you have a table with name "Foo" and with an index of {"bar", "baz"}, generated index name will be "index_Foo_bar_baz". If you need to specify the index in a query, you should never rely on this name, instead, specify a name for your index.

Returns
String The name of the index.

orders

public Order[] orders ()

List of column sort orders in the Index.

The number of entries in the array should be equal to size of columns in value().

The default order of all columns in the index is Index.Order.ASC.

Note that there is no value in providing a sort order on a single-column index. Column sort order of an index are relevant on multi-column indices and specifically in those that are considered 'covering indices', for such indices specifying an order can have performance improvements on queries containing ORDER BY clauses. See SQLite documentation for details on sorting by index and the usage of the sort order by the query optimizer.

As an example, consider a table called 'Song' with two columns, 'name' and 'length'. If a covering index is created for it: CREATE INDEX `song_name_length` on `Song` (`name` ASC, `length` DESC), then a query containing an ORDER BY clause with matching order of the index will be able to avoid a table scan by using the index, but a mismatch in order won't. Therefore the columns order of the index should be the same as the most frequently executed query with sort order.

Returns
Order[] The list of column sort orders in the Index.

unique

public boolean unique ()

If set to true, this will be a unique index and any duplicates will be rejected.

Returns
boolean True if index is unique. False by default.

value

public String[] value ()

List of column names in the Index.

The order of columns is important as it defines when SQLite can use a particular index. See SQLite documentation for details on index usage in the query optimizer.

Returns
String[] The list of column names in the Index.