UNDF: UNDF-2026-000000385

elasticsearch-001: MMRResultDiversification O(n²) selectedDocRanks.contains

Classification

CWE: CWE-407 (Inefficient Algorithmic Complexity)
Severity: HIGH
Path: Hot query-time ranking path — every MMR diversified search request

Location

server/src/main/java/org/elasticsearch/search/diversification/mmr/MMRResultDiversification.java:63

Defect

List<Integer> selectedDocRanks = new ArrayList<>();
// ...
for (int x = 0; x < topDocsSize && ...; x++) {
    for (RankDoc doc : docs) {
        int docRank = doc.rank;
        if (selectedDocRanks.contains(docRank)) {   // O(n) ArrayList scan
            continue;
        }
        // ...
    }
    selectedDocRanks.add(thisMaxMMRDocRank);
}

The outer loop runs up to topDocsSize iterations; the inner loop runs docs.length iterations; inside the inner loop selectedDocRanks.contains(docRank) performs an O(selectedDocRanks.size()) linear scan.

Total complexity: O(topDocsSize × docs × selectedDocRanks) = O(n³) in the worst case, reducing to O(n²) for typical result windows.

For a 1000-doc result window with 100 selected docs, this is ~100,000 list-scans per query instead of ~100,000 O(1) set lookups.

Fix

Pre-build a HashSet<Integer> that is kept in sync with selectedDocRanks:

List<Integer> selectedDocRanks = new ArrayList<>();
Set<Integer> selectedDocRankSet = new HashSet<>();

// when adding:
selectedDocRanks.add(thisMaxMMRDocRank);
selectedDocRankSet.add(thisMaxMMRDocRank);

// in the guard:
if (selectedDocRankSet.contains(docRank)) {   // O(1)
    continue;
}

Complexity

Metric	Before	After
contains()	O(n)	O(1)
Full loop	O(n²)–O(n³)	O(n²)
At n=1000	~500,000 comparisons	~1,000

Affected Versions

All versions that include MMRResultDiversification (introduced with semantic MMR ranking feature).

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