Relevance Scoring

When you run full-text search queries (such as MATCH_ANY, MATCH_ALL, MATCH_PHRASE, and so on), you often need to sort results by "match degree" so that the most relevant records are returned first. Doris provides Relevance Scoring: each row is given a numeric score based on the query conditions, and a higher score indicates stronger relevance.

Doris currently uses the BM25 (Best Matching 25) algorithm as the default implementation for relevance scoring.

Typical use cases:

• Search engine applications: return the most relevant document list based on keywords
• Log analysis: sort by relevance to quickly locate the logs that best match the keywords
• Content recommendation: filter priority content based on text similarity
• Knowledge base retrieval: return candidate answers ranked by question relevance

Quick Start

The following example shows how to enable relevance scoring in a query and sort by score:

SELECT *,
       score() AS relevance
FROM search_demo
WHERE content MATCH_ANY 'text search test'
ORDER BY relevance DESC
LIMIT 10;

After execution, Doris computes the score for each row using the BM25 algorithm and returns the top 10 most relevant records:

+------+-----------------------------------+---------+--------------+-----------+
| id   | content                           | author  | publish_date | relevance |
+------+-----------------------------------+---------+--------------+-----------+
|    1 | Full text search engine test demo | Alice   | 2024-01-01   |  2.915228 |
|    7 | Text processing techniques        | Grace   | 2024-01-07   |  1.341931 |
|    5 | Performance test framework        | Eve     | 2024-01-05   |  1.341931 |
|    3 | Advanced search algorithms        | Charlie | 2024-01-03   |  1.341931 |
+------+-----------------------------------+---------+--------------+-----------+

Activation Conditions

For score calculation to take effect, the query must meet all three of the following conditions:

1. The SELECT clause explicitly calls the score() function.
2. The WHERE clause contains at least one MATCH_* condition.
3. The query is a Top-N query, and ORDER BY is based on the score() result.

Supported Index Types

Index Type	Scoring Supported	Description
Tokenized inverted index	Yes	Can compute BM25 relevance scores
Non-tokenized inverted index	No	Supports only exact match; does not compute scores

Supported Query Types

• MATCH_ANY
• MATCH_ALL
• MATCH_PHRASE
• MATCH_PHRASE_PREFIX
• SEARCH

BM25 Algorithm

BM25 is a text relevance algorithm based on a probabilistic model. It performs a weighted calculation on matching results by considering three factors together: term frequency (TF), inverse document frequency (IDF), and record length. Compared with the traditional TF-IDF model, BM25 offers better robustness and tunability, and effectively balances the score difference between long and short text.

Core Formula

score = IDF × (tf × (k1 + 1)) / (tf + k1 × (1 - b + b × |d| / avgdl))

The variables in the formula are defined as follows:

Variable	Meaning
tf	Number of occurrences of the query term in the current row
IDF	Inverse document frequency, which measures how rare the term is
|d|	Length of the current row (number of tokens after tokenization)
avgdl	Average length of all rows in the table
k1, b	Algorithm tuning parameters

Parameters

Parameter	Default	Description
k1	1.2	Controls how much term frequency influences the score
b	0.75	Controls the strength of record-length normalization
boost	1.0	Query-level weight factor

Auxiliary Statistics

IDF   = log(1 + (N - n + 0.5) / (n + 0.5))
avgdl = total_terms / total_rows

Where:

• N: total number of rows in the table
• n: number of rows that contain the query term

When a query contains multiple terms, the final score is the sum of the scores of each term.

Interpreting the Results

Understanding the scoring results helps you use relevance ranking more accurately:

Characteristic	Description
Score range	Positive number with no fixed upper bound; usually only relative magnitude matters, not the absolute value
Multi-term query	When multiple terms are involved, the total score equals the sum of the per-term scores
Length effect	Given the same matching terms, a shorter record receives a slightly higher score
No matching term	If the query term does not appear in the table, the corresponding score is 0

FAQ

Q1: Why does my query not return scores?

Check the following in order:

1. Whether the score() function is called in SELECT.
2. Whether the WHERE clause contains a MATCH_* condition.
3. Whether ORDER BY is based on score().
4. Whether the index is a tokenized inverted index.

Q2: What is the difference between BM25 and TF-IDF?

BM25 builds on TF-IDF by introducing two improvements: term frequency saturation and length normalization. These make the scores of long and short text more balanced and provide better robustness.

Q3: Can scores be compared across queries?

This is not recommended. BM25 scores depend on the specific query terms and the data distribution in the table. Absolute values are not comparable between different queries. Compare relative magnitudes only within the result set of the same query.