RewriteQueriesToDropUnusedColumns.kt
/*
* Copyright 2020 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
/**
* When present, [RewriteQueriesToDropUnusedColumns] annotation will cause Room to
* rewrite your [Query] methods such that only the columns that are used in the response are
* queried from the database.
*
* This annotation is useful if you don't need all columns returned in a query but also don't
* want to spell out their names in the query projection.
*
* For example, if you have a `User` class with 10 fields and want to return only
* the `name` and `lastName` fields in a POJO, you could write the query like this:
*
* ```
* @Dao
* interface MyDao {
* @Query("SELECT * FROM User")
* public fun getAll(): List<NameAndLastName>
* }
*
* data class NameAndLastName (
* val name: String,
* val lastName: String
* )
* ```
*
* Normally, Room would print a [RoomWarnings.CURSOR_MISMATCH] warning since the query result
* has additional columns that are not used in the response. You can annotate the method with
* [RewriteQueriesToDropUnusedColumns] to inform Room to rewrite your query at compile time to
* avoid fetching extra columns.
*
* ```
* @Dao
* interface MyDao {
* @RewriteQueriesToDropUnusedColumns
* @Query("SELECT * FROM User")
* fun getAll(): List<NameAndLastName>
* }
* ```
*
* At compile time, Room will convert this query to `SELECT name, lastName FROM (SELECT *
* FROM User)` which gets flattened by **Sqlite** to `SELECT name, lastName FROM User`.
*
* When the annotation is used on a [Dao] method annotated with [Query], it will only
* affect that query. You can put the annotation on the [Dao] annotated class/interface or
* the [Database] annotated class where it will impact all methods in the dao / database
* respectively.
*
* Note that Room will not rewrite the query if it has multiple columns that have the same name as
* it does not yet have a way to distinguish which one is necessary.
*/
@Target(AnnotationTarget.FUNCTION, AnnotationTarget.CLASS)
@Retention(AnnotationRetention.BINARY)
public annotation class RewriteQueriesToDropUnusedColumns