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Group By & Aggregation

Learn how to group search results by metadata keys and select the top results from each group. GroupBy is useful for diversifying results, deduplication, and category-aware ranking.
GroupBy currently requires a ranking expression to be specified. Support for grouping without ranking is planned for a future release.

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How Grouping Works

GroupBy organizes ranked results into groups based on metadata keys, then performs aggregation on each group. Currently, aggregation supports MinK and MaxK, which select the top k results from each group based on the specified sorting keys. After grouping and aggregation, results from all groups are flattened and sorted by score. The limit() method operates on this flattened list. 
from chromadb import Search, K, Knn, GroupBy, MinK

# Get top 3 results per category, ordered by score
search = (Search()
    .rank(Knn(query="machine learning research"))
    .group_by(GroupBy(
        keys=K("category"),
        aggregate=MinK(keys=K.SCORE, k=3)
    ))
    .limit(30)
    .select(K.DOCUMENT, K.SCORE, "category"))

results = collection.search(search)

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The GroupBy Class

The GroupBy class specifies how to partition results and which records to keep from each partition. 
from chromadb import GroupBy, MinK, K

# Single grouping key
GroupBy(
    keys=K("category"),
    aggregate=MinK(keys=K.SCORE, k=3)
)

# Multiple grouping keys
GroupBy(
    keys=[K("category"), K("year")],
    aggregate=MinK(keys=K.SCORE, k=1)
)

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GroupBy Parameters

ParameterTypeDescription
keysKey or List[Key]Metadata key(s) to group by
aggregateMinK or MaxKAggregation function to select top k records within each group

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Aggregation Functions

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MinK

Keeps the k records with the smallest values for the specified keys. Use MinK when lower values are better (e.g., distance scores, prices, priorities). 
from chromadb import MinK, K

# Keep 3 records with lowest scores per group
MinK(keys=K.SCORE, k=3)

# Keep 2 records with lowest priority, then lowest score as tiebreaker
MinK(keys=[K("priority"), K.SCORE], k=2)
ParameterTypeDescription
keysKey or List[Key]Key(s) to sort by in ascending order
kintNumber of records to keep from each group

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MaxK

Keeps the k records with the largest values for the specified keys. Use MaxK when higher values are better (e.g., ratings, relevance scores, dates). 
from chromadb import MaxK, K

# Keep 3 records with highest ratings per group
MaxK(keys=K("rating"), k=3)

# Keep 2 records with highest year, then highest rating as tiebreaker
MaxK(keys=[K("year"), K("rating")], k=2)
ParameterTypeDescription
keysKey or List[Key]Key(s) to sort by in descending order
kintNumber of records to keep from each group

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Key References

Use K.SCORE to reference the search score, or K("field_name") for metadata fields. 
from chromadb import K

# Built-in score key
K.SCORE  # References "#score" - the search/ranking score

# Metadata field keys
K("category")   # References the "category" metadata field
K("priority")   # References the "priority" metadata field
K("year")       # References the "year" metadata field

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Common Patterns

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Single Key Grouping

Group by one metadata field and keep the top results from each group. 
# Top 2 articles per category by relevance
search = (Search()
    .rank(Knn(query="climate change impacts"))
    .group_by(GroupBy(
        keys=K("category"),
        aggregate=MinK(keys=K.SCORE, k=2)
    ))
    .limit(20))

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Multiple Key Grouping

Group by combinations of metadata fields for finer-grained control. 
# Top 1 article per (category, year) combination
search = (Search()
    .rank(Knn(query="renewable energy"))
    .group_by(GroupBy(
        keys=[K("category"), K("year")],
        aggregate=MinK(keys=K.SCORE, k=1)
    ))
    .limit(30))

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Multiple Ranking Keys with Tiebreakers

Sort within groups by multiple criteria when the primary key has ties. 
# Top 2 per category: sort by priority first, then by score
search = (Search()
    .rank(Knn(query="artificial intelligence"))
    .group_by(GroupBy(
        keys=K("category"),
        aggregate=MinK(keys=[K("priority"), K.SCORE], k=2)
    ))
    .limit(20))

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Dictionary Syntax

You can also construct GroupBy using dictionary syntax for programmatic query building. 
# GroupBy with MinK
search = Search(
    rank={"$knn": {"query": "machine learning", "limit": 100}},
    group_by={
        "keys": ["category"],
        "aggregate": {"$min_k": {"keys": ["#score"], "k": 3}}
    },
    limit=30
)

# GroupBy with MaxK
search = Search(
    rank={"$knn": {"query": "top rated products", "limit": 100}},
    group_by={
        "keys": ["category", "brand"],
        "aggregate": {"$max_k": {"keys": ["rating", "#score"], "k": 2}}
    },
    limit=20
)

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Edge Cases and Important Behavior

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Groups with Fewer Records

If a group has fewer records than the requested k, all records from that group are returned. 
# Request top 5 per category, but "rare_category" only has 2 documents
# Result: "rare_category" returns 2, other categories return up to 5
search = (Search()
    .rank(Knn(query="search query"))
    .group_by(GroupBy(keys=K("category"), aggregate=MinK(keys=K.SCORE, k=5)))
    .limit(50))

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Missing Metadata Keys

Documents missing the grouping key are treated as having a null/None value for that key, and are grouped together.

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Limit Still Applies

The Search.limit() still controls the final number of results returned after grouping. Set it high enough to include results from all groups.

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Complete Example

Here’s a practical example showing diversified search results across categories: 
from chromadb import Search, K, Knn, GroupBy, MinK

# Diversified product search - ensure results from multiple categories
search = (Search()
    .where(K("in_stock") == True)
    .rank(Knn(query="wireless headphones", limit=100))
    .group_by(GroupBy(
        keys=K("category"),
        aggregate=MinK(keys=K.SCORE, k=2)  # Top 2 per category
    ))
    .limit(20)
    .select(K.DOCUMENT, K.SCORE, "name", "category", "price"))

results = collection.search(search)
rows = results.rows()[0]

# Results now include top 2 from each category instead of
# potentially all results from a single dominant category
for row in rows:
    print(f"{row['metadata']['name']}")
    print(f"  Category: {row['metadata']['category']}")
    print(f"  Price: ${row['metadata']['price']:.2f}")
    print(f"  Score: {row['score']:.3f}")
    print()

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Tips and Best Practices

  • Set Knn limit high enough - The Knn limit determines the candidate pool before grouping. Set it high enough to include candidates from all groups you want represented.
  • Use MinK with scores - Since Chroma uses distance-based scoring (lower is better), use MinK with K.SCORE to get the most relevant results per group.
  • Use MaxK for user-defined metrics - For metadata fields where higher is better (ratings, popularity), use MaxK.
  • Combine with filtering - Use .where() to filter before grouping to reduce the candidate pool to relevant documents.
  • Account for group size variance - Groups may return fewer than k results if they don’t have enough matching documents.

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Next Steps