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Measure Recall ​

In the world of vector search, recall refers to the percentage of vectors that the index returns which are true nearest neighbors. For example, if a nearest neighbor query for the 200 nearest neighbors returns 194 of the ground truth nearest neighbors, then the recall is 194/200 x 100 = 97%.

In a vector query, recall is important because it measures the percentage of relevant results retrieved from a search. Recall helps you evaluate the quality of a vector index and provides insight into balancing search speed and accuracy.

With VectorChord, you can find the recall for a vector query on a vector index for any SQL query. You can easily tune the search parameters to achieve the desired search recall.

sql
-- You can tune the search parameters before measure
-- SET vchordrq.probes = '100'
-- SET vchordrq.epsilon = 1.0

SELECT vchordrq_evaluate_query_recall(query=>$$
  SELECT * FROM items ORDER BY embedding <-> '[3,1,2]' LIMIT 10
$$);
sql
-- You can tune the search parameters before measure
-- SET vchordrq.probes = '100'
-- SET vchordrq.epsilon = 1.0

-- Fast evaluate for vchordg is not implemented yet, it's a workaround to use
SELECT vchordrq_evaluate_query_recall(query=>$$
  SELECT * FROM items ORDER BY embedding <-> '[3,1,2]' LIMIT 10
$$, exact_search=>true);
Details

By default, the function uses exact_search=>false to generate a slightly inaccurate ground truth by setting vchordrq.probes to an extremely large value (65535). This method is much faster than using exact_search=true for a full table scan, and the resulting precision is acceptable in most situations.

Reference ​

Functions vchordrq ​

vchordrq_evaluate_query_recall ​

  • Description: Evaluates the recall of a given SQL query.
  • Result: real (a value between 0.0 and 1.0, or NaN if no results are found)
  • Arguments:
    • query(text): The SQL query to be evaluated.
    • exact_search(boolean): A flag to indicate whether an full table scan should be performed for the ground truth set. The default value is false.
    • accu_probes(text): Used when exact_search is false. It specifies the vchordrq.probes value for the ANN search that generates the estimated ground truth. If NULL, it will be derived from the active vchordrq.probes setting during the initial query execution.
    • accu_epsilon(real): Used when exact_search is false. It specifies the vchordrq.epsilon value for the ANN search that generates the estimated ground truth. The default value is 1.9.
  • Example:
    • SELECT vchordrq_evaluate_query_recall(query=>$$SELECT ctid FROM t ORDER BY val <-> '[0.5, 0.25, 1.0]' LIMIT 10$$);
    • SELECT vchordrq_evaluate_query_recall(query=>$$SELECT ctid FROM t ORDER BY val <-> '[0.5, 0.25, 1.0]' LIMIT 10$$, exact_search=>true);
    • SELECT vchordrq_evaluate_query_recall(query=>$$SELECT ctid FROM t ORDER BY val <-> '[0.5, 0.25, 1.0]' LIMIT 10$$, exact_search=>true, accu_probes=>'100', accu_epsilon=>3.9);