RewriteQueriesToDropUnusedColumns.java
/*
* 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;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* When present, {@link RewriteQueriesToDropUnusedColumns} annotation will cause Room to
* rewrite your {@link Query} methods such that only the columns that are used in the response are
* queried from the database.
* <p>
* 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.
* <p>
* For example, if you have a {@code User} class with 10 fields and want to return only
* the {@code name} and {@code lastName} fields in a POJO, you could write the query like this:
*
* <pre>
* {@literal @}Dao
* interface MyDao {
* {@literal @Query("SELECT * FROM User")}
* {@literal public List<NameAndLastName> getAll();}
* }
* class NameAndLastName {
* public String name;
* public String lastName;
* }
* </pre>
* <p>
* Normally, Room would print a {@link RoomWarnings#CURSOR_MISMATCH} warning since the query result
* has additional columns that are not used in the response. You can annotate the method with
* {@link RewriteQueriesToDropUnusedColumns} to inform Room to rewrite your query at compile time to
* avoid fetching extra columns.
* <pre>
* {@literal @}Dao
* interface MyDao {
* {@literal @RewriteQueriesToDropUnusedColumns}
* {@literal @Query("SELECT * FROM User")}
* {@literal public List<NameAndLastName> getAll();}
* }
* </pre>
* At compile time, Room will convert this query to {@code SELECT name, lastName FROM (SELECT *
* FROM User)} which gets flattened by <b>Sqlite</b> to {@code SELECT name, lastName FROM User}.
* <p>
* When the annotation is used on a {@link Dao} method annotated with {@link Query}, it will only
* affect that query. You can put the annotation on the {@link Dao} annotated class/interface or
* the {@link Database} annotated class where it will impact all methods in the dao / database
* respectively.
* <p>
* 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({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.CLASS)
public @interface RewriteQueriesToDropUnusedColumns {
}