TopnToMax.java

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package org.apache.doris.nereids.rules.expression.rules;

import org.apache.doris.nereids.rules.expression.ExpressionPatternMatcher;
import org.apache.doris.nereids.rules.expression.ExpressionPatternRuleFactory;
import org.apache.doris.nereids.rules.expression.ExpressionRuleType;
import org.apache.doris.nereids.trees.expressions.Expression;
import org.apache.doris.nereids.trees.expressions.functions.agg.Max;
import org.apache.doris.nereids.trees.expressions.functions.agg.TopN;
import org.apache.doris.nereids.trees.expressions.literal.IntegerLikeLiteral;

import com.google.common.collect.ImmutableList;

import java.util.List;

/**
 * Convert topn(x, 1) to max(x)
 */
public class TopnToMax implements ExpressionPatternRuleFactory {

    public static final TopnToMax INSTANCE = new TopnToMax();

    @Override
    public List<ExpressionPatternMatcher<? extends Expression>> buildRules() {
        return ImmutableList.of(
                matchesTopType(TopN.class).then(TopnToMax::rewrite)
                        .toRule(ExpressionRuleType.TOPN_TO_MAX)
        );
    }

    /** rewrite */
    public static Expression rewrite(TopN topN) {
        if (topN.arity() == 2 && topN.child(1) instanceof IntegerLikeLiteral
                && ((IntegerLikeLiteral) topN.child(1)).getIntValue() == 1) {
            return new Max(topN.child(0));
        } else {
            return topN;
        }
    }
}