Index.kt
/*
* 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
/**
* Declares an index on an [Entity].
* see: [SQLite Index Documentation](https://sqlite.org/lang_createindex.html)
*
* 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`.
* */
@Target(allowedTargets = []) // Complex annotation target
@Retention(AnnotationRetention.BINARY)
public annotation class Index(
/**
* 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](https://www.sqlite.org/optoverview.html) for details on
* index usage in the query optimizer.
*
* @return The list of column names in the [Index].
*/
vararg val value: String,
/**
* 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 [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](https://www.sqlite.org/queryplanner.html#_sorting_by_index)
* 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.
*
* @return The list of column sort orders in the Index.
*/
val orders: Array<Order> = [],
/**
* 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.
*/
val name: String = "",
/**
* 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.
*/
val unique: Boolean = false
) {
public enum class Order {
/**
* Ascending returning order.
*
* @see Index.orders
*/
ASC,
/**
* Descending returning order.
*
* @see Index.orders
*/
DESC
}
}