Rerank In Table

To maximize performance, VectorChord stores both the quantized and original vectors in the index. Although the original vectors can be fetched from the table, doing so is much slower than fetching them from our highly optimized data structure.

As a result, VectorChord's disk usage could be larger than that of alternatives. This is usually acceptable because disk space is affordable and performance is more important.

This behavior is called rerank_in_index and is the default option for VectorChord.

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However, if you are short on disk space and don't care much about performance, you can enable rerank_in_table in CREATE INDEX to reduce disk usage.

CREATE INDEX ON items USING vchordrq (embedding vector_l2_ops) WITH (options = $$
residual_quantization = true
rerank_in_table = true
[build.internal]
lists = []
$$);

Then, all queries of this index will retrieve the original vectors from the table, which will significantly reduce disk usage.

Performance Trade-offs

• Compared to the rerank_in_index option, the rerank_in_table option will result in a 30-50% increase in latency and QPS loss at query
• Compared to the rerank_in_index option, the rerank_in_table option index will save about 50% of disk usage