UNDF: UNDF-2026-000000001

UNDF: (pending)

doris-0003: EquivalenceClass.getEquivalenceSetList — O(N²) List.contains dedup

CWE-407 — Algorithmic Complexity

Field	Value
ID	doris-0003
Severity	MEDIUM
Ecosystem	doris
Package	fe-core
File	`fe/fe-core/src/main/java/org/apache/doris/nereids/rules/exploration/mv/EquivalenceClass.java`
Lines	130–146
Complexity	O(N²)
Hot path	Called during materialized view rewrite in query planning (every query with MV candidates)

Defect

getEquivalenceSetList() deduplicates equivalence class lists using a List<List<SlotReference>> as the visited set. visited.contains(slotSet) calls ArrayList.contains() which iterates the entire visited list comparing lists element-by-element — O(N) per call. The outer loop also iterates N times, making the total complexity O(N²) where N is the number of distinct equivalence slot lists in equivalenceSlotMap.values().

This runs on every query that touches materialized view rewrite logic in Doris Nereids.

public List<List<SlotReference>> getEquivalenceSetList() {
    if (equivalenceSlotList != null) {
        return equivalenceSlotList;
    }
    List<List<SlotReference>> equivalenceSets = new ArrayList<>();
    List<List<SlotReference>> visited = new ArrayList<>();         // O(N) per contains
    equivalenceSlotMap.values().forEach(slotSet -> {
        if (!visited.contains(slotSet)) {                          // O(N) scan
            equivalenceSets.add(slotSet);
        }
        visited.add(slotSet);
    });
    this.equivalenceSlotList = equivalenceSets;
    return this.equivalenceSlotList;
}

Fix

equivalenceSlotMap maps each SlotReference to its equivalence-class list (which is a shared List<SlotReference> object). Deduplication by object identity is sufficient and correct because the union-find structure ensures all members of an equivalence class share the same list reference. Use IdentityHashMap or Set based on object identity:

public List<List<SlotReference>> getEquivalenceSetList() {
    if (equivalenceSlotList != null) {
        return equivalenceSlotList;
    }
    // Use identity comparison: all slots in the same equivalence class share the same List object
    Set<List<SlotReference>> seen = Collections.newSetFromMap(new IdentityHashMap<>());
    List<List<SlotReference>> equivalenceSets = new ArrayList<>();
    for (List<SlotReference> slotSet : equivalenceSlotMap.values()) {
        if (seen.add(slotSet)) {                                    // O(1) identity hash
            equivalenceSets.add(slotSet);
        }
    }
    this.equivalenceSlotList = equivalenceSets;
    return this.equivalenceSlotList;
}

Speedup

N (equivalence classes)	Before (ops)	After (ops)	Speedup
10	100	10	10×
50	2,500	50	50×
100	10,000	100	100×
500	250,000	500	500×

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