Index.java
/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.room;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* Declares an index on an Entity.
* see: <a href="https://sqlite.org/lang_createindex.html">SQLite Index Documentation</a>
* <p>
* 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.
* <p>
* There are 2 ways to define an index in an {@link Entity}. You can either set
* {@link ColumnInfo#index()} property to index individual fields or define composite indices via
* {@link Entity#indices()}.
* <p>
* If an indexed field is embedded into another Entity via {@link Embedded}, it is <b>NOT</b>
* added as an index to the containing {@link Entity}. If you want to keep it indexed, you must
* re-declare it in the containing {@link Entity}.
* <p>
* Similarly, if an {@link Entity} extends another class, indices from the super classes are
* <b>NOT</b> inherited. You must re-declare them in the child {@link Entity} or set
* {@link Entity#inheritSuperIndices()} to {@code true}.
* */
@Target({})
@Retention(RetentionPolicy.CLASS)
public @interface Index {
/**
* List of column names in the Index.
* <p>
* The order of columns is important as it defines when SQLite can use a particular index.
* See <a href="https://www.sqlite.org/optoverview.html">SQLite documentation</a> for details on
* index usage in the query optimizer.
*
* @return The list of column names in the Index.
*/
String[] value();
/**
* List of column sort orders in the Index.
* <p>
* The number of entries in the array should be equal to size of columns in {@link #value()}.
* <p>
* The default order of all columns in the index is {@link Order#ASC}.
* <p>
* 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.SchemaDifferTest See
* <a href="https://www.sqlite.org/queryplanner.html#_sorting_by_index">SQLite documentation</a>
* for details on sorting by index and the usage of the sort order by the query optimizer.
* <p>
* As an example, consider a table called 'Song' with two columns, 'name' and 'length'. If a
* covering index is created for it: <code>CREATE INDEX `song_name_length` on `Song`
* (`name` ASC, `length` DESC)</code>, 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.
*
* @return The list of column sort orders in the Index.
*/
Order[] orders() default {};
/**
* 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.
*
* @return The name of the index.
*/
String name() default "";
/**
* If set to true, this will be a unique index and any duplicates will be rejected.
*
* @return True if index is unique. False by default.
*/
boolean unique() default false;
enum Order {
/**
* Ascending returning order.
*
* @see Index#orders()
*/
ASC,
/**
* Descending returning order.
*
* @see Index#orders()
*/
DESC
}
}