RemoveUnusedColumnQueryRewriter.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.parser.optimization
import androidx.room.parser.ParsedQuery
import androidx.room.parser.SqlParser
import androidx.room.processor.QueryRewriter
import androidx.room.solver.query.result.QueryResultAdapter
/**
* If the query response has unused columns, this rewrites the query to only fetch those columns.
*
* e.g. if it is a query like `SELECT * FROM User` where only `name` and `lastName` columns are
* accessed in the generated code, this re-writer will change it to
* `SELECT name, lastName FROM (SELECT * FROM User)`. Sqlite takes care of the rest where it
* flattens the query to avoid fetching unused columns in intermediate steps.
*/
object RemoveUnusedColumnQueryRewriter : QueryRewriter {
override fun rewrite(query: ParsedQuery, resultAdapter: QueryResultAdapter): ParsedQuery {
// cannot do anything w/o a result info
val resultInfo = query.resultInfo ?: return query
if (resultAdapter.mappings.isEmpty()) {
return query
}
val usedColumns = resultAdapter.mappings.flatMap { it.usedColumns }
val columnNames = resultInfo.columns.map { it.name }
val unusedColumns = columnNames - usedColumns
if (unusedColumns.isEmpty()) {
return query // nothing to optimize here
}
if (columnNames.size != columnNames.distinct().size) {
// if result has duplicate columns, ignore for now
return query
}
val usedColumnNames = columnNames - unusedColumns
val updated = SqlParser.parse(
"SELECT ${usedColumnNames.joinToString(", ") { "`$it`" }} FROM (${query.original})"
)
if (updated.errors.isNotEmpty()) {
// we somehow messed up, return original
return query
}
return updated
}
}