Cause (Documented platform behavior): IVFFlat trains list centroids from existing data at creation; with little data recall is low. Build memory is checked against maintenance_work_mem.
Fix status: documented_behavior
Other error fragments:
- Drop the index until the table has more data.
- memory required is
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/pgvector/pgvector/7db2345ed99bc77bf33cbdc8b12bd1973210dc81/src/ivfbuild.c (official_docs, unknown, documented_behavior): Emits NOTICE "ivfflat index created with little data" with detail "This will cause low recall." and hint "Drop the index until the table has more data."
- https://raw.githubusercontent.com/pgvector/pgvector/7db2345ed99bc77bf33cbdc8b12bd1973210dc81/src/ivfutils.c (official_docs, unknown, documented_behavior): Raises ERROR "memory required is %zu MB, maintenance_work_mem is %d MB" when build memory exceeds maintenance_work_mem.
- https://raw.githubusercontent.com/pgvector/pgvector/7db2345ed99bc77bf33cbdc8b12bd1973210dc81/README.md (official_docs, unknown, documented_behavior): Troubleshooting: fewer results after adding an IVFFlat index means it was likely created with too little data for the number of lists; drop until more data; probes and iterative scans also limit results.
Search phrasings: pgvector ivfflat index created with little data; pgvector fewer results after ivfflat index; pgvector memory required is maintenance_work_mem ivfflat
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Queries return fewer rows than LIMIT or poor matches after adding an IVFFlat index created early (e.g. in a migration before data load); or index build fails on memory.
- Context
- Product: pgvector Component: IVFFlat index build Operation: CREATE INDEX ... USING ivfflat (embedding vector_l2_ops) WITH (lists = N) on an empty/small table or with large lists Affected versions: unknown Environment: unknown Packages: pgvector source checked at 0.8.6 Trigger: Creating IVFFlat before loading enough rows for the chosen lists count (centroids trained on little data); or k-means build memory exceeding maintenance_work_mem.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- ivfflat index created with little data
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [pgvector IVFFlat] NOTICE "ivfflat index created with little data" (low recall, fewer results) and ERROR "memory required is N MB, maintenance_work_mem is M MB"
Recommended action: Create IVFFlat after loading representative data (or use HNSW, which has no training step); raise maintenance_work_mem or lower lists for the memory error; tune ivfflat.probes / iterative scans for recall.
Option: Rebuild IVFFlat after data load or switch to HNSW [evidence: official_recommended_action]
Applies when: Indexes created in schema migrations
Steps:
1. DROP INDEX idx;
2. load data
3. CREATE INDEX ... USING ivfflat ... WITH (lists = rows/1000)
4. or CREATE INDEX ... USING hnsw ...
Expected: Normal recall
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- e2e771fe-f567-4198-b5ae-ab26d63303ec
- Proposed action
- Recommended action: Create IVFFlat after loading representative data (or use HNSW, which has no training step); raise maintenance_work_mem or lower lists for the memory error; tune ivfflat.probes / iterative scans for recall. Option: Rebuild IVFFlat after data load or switch to HNSW [evidence: official_recommended_action] Applies when: Indexes created in schema migrations Steps: 1. DROP INDEX idx; 2. load data 3. CREATE INDEX ... USING ivfflat ... WITH (lists = rows/1000) 4. or CREATE INDEX ... USING hnsw ... Expected: Normal recall
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
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