LimitStatsRule.java

/*
 * 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.facebook.presto.cost;

import com.facebook.presto.Session;
import com.facebook.presto.matching.Pattern;
import com.facebook.presto.spi.plan.LimitNode;
import com.facebook.presto.sql.planner.TypeProvider;
import com.facebook.presto.sql.planner.iterative.Lookup;

import java.util.Optional;

import static com.facebook.presto.sql.planner.plan.Patterns.limit;

public class LimitStatsRule
        extends SimpleStatsRule<LimitNode>
{
    private static final Pattern<LimitNode> PATTERN = limit();

    public LimitStatsRule(StatsNormalizer normalizer)
    {
        super(normalizer);
    }

    @Override
    public Pattern<LimitNode> getPattern()
    {
        return PATTERN;
    }

    @Override
    protected Optional<PlanNodeStatsEstimate> doCalculate(LimitNode node, StatsProvider statsProvider, Lookup lookup, Session session, TypeProvider types)
    {
        PlanNodeStatsEstimate sourceStats = statsProvider.getStats(node.getSource());
        if (sourceStats.getOutputRowCount() <= node.getCount()) {
            return Optional.of(sourceStats);
        }

        // LIMIT actually limits (or when there was no row count estimated for source)
        return Optional.of(PlanNodeStatsEstimate.buildFrom(sourceStats)
                .setOutputRowCount(node.getCount())
                .setConfidence(sourceStats.confidenceLevel())
                .build());
    }
}