Pre And Post Filtering since v0.4.0
In VectorChord, the most costly step in a vector search is rerank, which computes vector distances precisely.
For a vector search with a filter, there are two ways to handle the interaction of filter and rerank:
Pre-filtering: Apply the filter before the rerank step.
Post-filtering: Apply the filter after the rerank step.
For a very tight (highly selective) filter, if only 1% of the rows may be selected. In this case, pre-filtering can reduce the number of reranks required by about 100 times, thus significantly improving query performance.
TIP
Post-filtering is the default behavior. To switch to Pre-Filtering, set vchordrq.prefilter=True before query.
Performance Trade-offs
If the WHERE clause is highly selective, pre-filtering is more efficient, as it reduces the number of candidates that need reranking.
If the WHERE clause is not very highly selective, post-filtering may be more efficient, as it avoids the overhead of checking filter conditions on many candidates that may not make it to the final results.
| Example | All rows | Selected rows | Select rate |
|---|---|---|---|
| A low selective filter | 1000 | 900 | 90% |
| A medium selective filter | 1000 | 300 | 30% |
| A highly selective filter | 1000 | 10 | 1% |
WARNING
Pre-filtering only support conditions on the same table yet.
Based on our experimental results, the QPS speedup at different select rate is as follows:
- 200% speedup at a select rate of 1%
- Not significant (5%) speedup at a select rate of 10%
