room/room-common/src/commonMain/kotlin/androidx/room/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
    }
}