Fix join set degeneration on low-degree nodes - #16435
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mayya-sharipova merged 2 commits intoJul 30, 2026
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UpdateGraphsUtils#computeJoinSet asked for a coverage of 2 for every node with a degree below 9, including nodes with a degree of 1. Such a node can be covered at most once (only its neighbours can cover it), so the coverage was unsatisfiable and the node always ended up in the join set itself. On hub-and-spoke shaped graphs, which HNSW produces for 1-dimensional or heavily duplicated vectors, almost all nodes have a degree of 1, so the join set degenerated to the whole graph and merging lost all its savings. Clamp the requested coverage by the node's degree; graphs where all nodes have a degree of 2 or more are unaffected. Fixes apache#16251
The fix is going to be backported to branch_10x, so it will ship in 10.6.0 rather than 11.0.0.
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UpdateGraphsUtils#computeJoinSet asked for a coverage of 2 for every node with a degree below 9, including nodes with a degree of 1. Such a node can be covered at most once (only its neighbours can cover it), so the coverage was unsatisfiable and the node always ended up in the join set itself. On hub-and-spoke shaped graphs, which HNSW produces for 1-dimensional or heavily duplicated vectors, almost all nodes have a degree of 1, so the join set degenerated to the whole graph and merging lost all its savings. Clamp the requested coverage by the node's degree; graphs where all nodes have a degree of 2 or more are unaffected. Fixes #16251
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UpdateGraphsUtils#computeJoinSet asked for a coverage of 2 for every node with a degree below 9, including nodes with a degree of 1. Such a node can be covered at most once (only its neighbours can cover it), so the coverage was unsatisfiable and the node always ended up in the join set itself.
On hub-and-spoke shaped graphs, which HNSW produces for 1-dimensional or heavily duplicated vectors, almost all nodes have a degree of 1, so the join set degenerated to the whole graph and merging lost all its savings. Clamp the requested coverage by the node's degree; graphs where all nodes have a degree of 2 or more are unaffected.
Fixes #16251