com.zeoflow.depot.parser.optimization.RemoveUnusedColumnQueryRewriter.kt Maven / Gradle / Ivy

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The Depot persistence library provides an abstraction layer over SQLite to allow for more robust database access while using the full power of SQLite.

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/*
 * Copyright (C) 2021 ZeoFlow SRL
 *
 * 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 com.zeoflow.depot.parser.optimization

import com.zeoflow.depot.parser.ParsedQuery
import com.zeoflow.depot.parser.SqlParser
import com.zeoflow.depot.processor.QueryRewriter
import com.zeoflow.depot.solver.query.result.PojoRowAdapter
import com.zeoflow.depot.solver.query.result.RowAdapter

/**
 * 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, rowAdapter: RowAdapter): ParsedQuery {
        if (rowAdapter !is PojoRowAdapter) {
            return query
        }
        // cannot do anything w/o a result info
        val resultInfo = query.resultInfo ?: return query

        val unusedColumns = rowAdapter.mapping.unusedColumns
        if (unusedColumns.isEmpty()) {
            return query // nothing to optimize here
        }
        val columnNames = resultInfo.columns.map { it.name }
        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
    }
}