Relevancy

Relevancy is a term referring to the accuracy and effectiveness of search results. If search results are almost always appropriate, then they can be considered relevant, and vice versa.

MeiliSearch has a number of features for fine-tuning the relevancy of search results. The most important tool among them is ranking rules.

Ranking rules

In order to ensure relevant results, search responses are sorted based on a set of consecutive rules called ranking rules.

Behavior

Each index possesses a list of ranking rules stored as an array in the settings object. This array is fully customizable, meaning you can delete existing rules, add new ones, and reorder them as needed.

Whenever a search query is made, MeiliSearch uses a bucket sort algorithm to rank documents. The first ranking rule is applied to all documents, while each subsequent rule is only applied to documents that are considered equal under the previous rule (i.e. as a tiebreaker).

The order in which ranking rules are applied matters. The first rule in the array has the most impact, and the last rule has the least. Our default configuration meets most standard needs but you can change it.

Built-in rules

MeiliSearch contains six built-in ranking rules in the following order:

1. Words
2. Typo
3. Proximity
4. Attribute
5. Sort
6. Exactness

1. Words

Results are sorted by decreasing number of matched query terms. Returns documents that contain all query terms first.

NOTE

The words rule works from right to left. Therefore, the order of the query string impacts the order of results.

For example, if someone were to search batman dark knight, then the words rule would rank documents containing all three terms first, documents containing only batman and dark second, and documents containing only batman third.

2. Typo

Results are sorted by increasing number of typos. Returns documents that match query terms with fewer typos first.

3. Proximity

Results are sorted by increasing distance between matched query terms. Returns documents where query terms occur close together and in the same order as the query string first.

4. Attribute

Results are sorted according to the attribute ranking order. Returns documents that contain query terms in more important attributes first.

Also, note the documents with attributes containing the query words at the beginning of the attribute will be considered more relevant than documents containing the query words at the end of the attributes.

5. Sort

Results are sorted according to parameters decided at query time. When the sort ranking rule is in a higher position, sorting is exhaustive: results will be less relevant, but follow the user-defined sorting order more closely. When sort is in a lower position, sorting is relevant: results will be very relevant, but might not always follow the order defined by the user.

NOTE

Differently from other ranking rules, sort is only active for queries containing the sort search parameter (opens new window). If a search request does not contain sort or if its value is invalid, this rule will be ignored.

6. Exactness

Results are sorted by the similarity of the matched words with the query words. Returns documents that contain exactly the same terms as the ones queried first.

Examples

Typo

Proximity

Attribute

Exactness

Typo

• vogli: 0 typo
• volli: 1 typo

The typo rule sorts the results by increasing number of typos on matched query words.

Proximity

The reason why Creature is listed before Mississippi Grind is because of the proximity rule. The smallest distance between the matching words in creature is smaller than the smallest distance between the matching words in Mississippi Grind.

The proximity rule sorts the results by increasing distance between matched query terms.

Attribute

If It's Tuesday, This must be Belgium is the first document because the matched word Belgium, is found in the title attribute and not the description.

The attribute rule sorts the results by attribute importance.

Exactness

Knight Moves is displayed before Knights of Badassdom. Knight is exactly the same as the search query Knight whereas there is a letter of difference between Knights and the search query Knight.

Custom rules

For now, MeiliSearch supports two custom rules that can be added to the ranking rules array: one for ascending sort and one for descending sort.

To add a custom ranking rule, you have to communicate the attribute name followed by a colon (:) and either asc for ascending order or desc for descending order.

• To apply an ascending sort (results sorted by increasing value of the attribute): attribute_name:asc

• To apply a descending sort (results sorted by decreasing value of the attribute): attribute_name:desc

The attribute must have either a numeric or a string value in all of the documents contained in that index.

Add this rule to the existing list of ranking rules using the update ranking rules endpoint.

Example

Let’s say you have a movie dataset. The documents contain the fields release_date with a timestamp as value, and movie_ranking an integer that represents its ranking.

The following example will create a rule that makes older movies more relevant than recent ones. A movie released in 1999 will appear before a movie released in 2020.

release_date:asc

The following example will create a rule that makes movies with a good rank more relevant than movies with a lower rank. Movies with a higher ranking will appear first.

movie_ranking:desc

To add a rule to the existing ranking rule, you have to add the rule to the existing ordered rules array using the settings route,

[
  "words",
  "typo",
  "proximity",
  "attribute",
  "sort",
  "exactness",
  "release_date:asc",
  "movie_ranking:desc"
]

Sorting and custom ranking rules

MeiliSearch allows users to define sorting order at query time by using the sort search parameter. There is some overlap between sorting and custom ranking rules, but the two do have different uses.

In general, sort will be most useful when you want to allow users to define what type of results they want to see first. A good use-case for sort is creating a webshop interface where customers can sort products by descending or ascending product price.

Custom ranking rules, instead, are always active after configured and will be useful when you want to promote certain types of results. A good use-case for custom ranking rules is ensuring discounted products in a webshop always feature among the top results.

Default order

By default, the built-in rules are executed in the following order.

[
  "words", 
  "typo", 
  "proximity", 
  "attribute", 
  "sort", 
  "exactness"
]

Depending on your needs, you might want to change this order of importance. To do so, you can use the update ranking rules endpoint.

Attribute ranking order

In a typical dataset, some fields are more relevant to search than others. A title, for example, has a value more meaningful to a movie search than its description or its release_date.

By default, the attribute ranking order is generated automatically based on the attributes’ order of appearance in the indexed documents. However, it can also be set manually.

For a more detailed look at this subject, see our reference page for the searchable attributes list.

Example

[
  "title", 
  "description", 
  "release_date"
]

With the above attribute ranking order, matching words found in the title field would have a higher impact on relevancy than the same words found in description or release_date. If you searched “1984”, for example, results like Michael Radford’s film “1984” would be ranked higher than movies released in the year 1984.