LogicalTopNToPhysicalTopN.java

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

import org.apache.doris.nereids.exceptions.AnalysisException;
import org.apache.doris.nereids.rules.Rule;
import org.apache.doris.nereids.rules.RuleType;
import org.apache.doris.nereids.trees.plans.Plan;
import org.apache.doris.nereids.trees.plans.SortPhase;
import org.apache.doris.nereids.trees.plans.logical.LogicalTopN;
import org.apache.doris.nereids.trees.plans.physical.PhysicalTopN;
import org.apache.doris.nereids.util.Utils;
import org.apache.doris.qe.ConnectContext;

import com.google.common.collect.Lists;

import java.util.List;

/**
 * Implementation rule that convert logical top-n to physical top-n.
 */
public class LogicalTopNToPhysicalTopN extends OneImplementationRuleFactory {
    @Override
    public Rule build() {
        return logicalTopN().thenApplyMulti(ctx -> twoPhaseSort(ctx.root))
                .toRule(RuleType.LOGICAL_TOP_N_TO_PHYSICAL_TOP_N_RULE);
    }

    /**
     * before: logicalTopN(off, limit)
     * after:
     *     gatherTopN(limit, off, require gather)
     *     mergeTopN(limit, off, require gather) -> localTopN(off+limit, 0, require any)
     */
    private List<PhysicalTopN<? extends Plan>> twoPhaseSort(LogicalTopN<? extends Plan> logicalTopN) {
        int sortPhaseNum = 0;
        if (ConnectContext.get() != null) {
            sortPhaseNum = ConnectContext.get().getSessionVariable().sortPhaseNum;
        }
        // The two-phase local sort keeps limit + offset rows. When that overflows the long range
        // (e.g. LIMIT and OFFSET both BIGINT_MAX), only the single-phase gather sort is safe: it
        // applies limit and offset separately and cannot overflow. The two-phase MERGE_SORT cannot be
        // used because ChildrenPropertiesRegulator forbids inserting a gather Exchange under
        // GATHER_SORT, so the optimizer cannot produce a valid distributed plan with only one-phase.
        // If sort_phase_num != 1 (i.e. the user did not explicitly request single-phase), report the
        // error so the user knows to set sort_phase_num = 1 for this extreme boundary.
        if (Utils.addOverflows(logicalTopN.getLimit(), logicalTopN.getOffset())) {
            if (sortPhaseNum != 1) {
                throw new AnalysisException("limit + offset overflows the long range in two-phase"
                        + " sort; set sort_phase_num = 1 to use single-phase sort for this query");
            }
            PhysicalTopN<Plan> onePhaseSort = new PhysicalTopN<>(logicalTopN.getOrderKeys(),
                    logicalTopN.getLimit(), logicalTopN.getOffset(), SortPhase.GATHER_SORT,
                    logicalTopN.getLogicalProperties(), logicalTopN.child(0));
            return Lists.newArrayList(onePhaseSort);
        }
        PhysicalTopN<Plan> localSort = new PhysicalTopN<>(logicalTopN.getOrderKeys(),
                logicalTopN.getLimit() + logicalTopN.getOffset(), 0, SortPhase.LOCAL_SORT,
                logicalTopN.getLogicalProperties(), logicalTopN.child(0));
        if (sortPhaseNum == 1) {
            PhysicalTopN<Plan> onePhaseSort = new PhysicalTopN<>(logicalTopN.getOrderKeys(), logicalTopN.getLimit(),
                    logicalTopN.getOffset(), SortPhase.GATHER_SORT,
                    logicalTopN.getLogicalProperties(), localSort.child(0));
            return Lists.newArrayList(onePhaseSort);
        } else if (sortPhaseNum == 2) {
            PhysicalTopN<Plan> twoPhaseSort = new PhysicalTopN<>(logicalTopN.getOrderKeys(), logicalTopN.getLimit(),
                    logicalTopN.getOffset(), SortPhase.MERGE_SORT, logicalTopN.getLogicalProperties(), localSort);
            return Lists.newArrayList(twoPhaseSort);
        } else {
            PhysicalTopN<Plan> twoPhaseSort = new PhysicalTopN<>(logicalTopN.getOrderKeys(), logicalTopN.getLimit(),
                    logicalTopN.getOffset(), SortPhase.MERGE_SORT, logicalTopN.getLogicalProperties(), localSort);
            PhysicalTopN<Plan> onePhaseSort = new PhysicalTopN<>(logicalTopN.getOrderKeys(), logicalTopN.getLimit(),
                    logicalTopN.getOffset(), SortPhase.GATHER_SORT,
                    logicalTopN.getLogicalProperties(), localSort.child(0));
            return Lists.newArrayList(twoPhaseSort, onePhaseSort);
        }
    }
}