Synonyms can create excessively complex search queries

[loiclec]

When a word has a multi-word synonym, for example:

"btw" -> "by the way"

or, worse, multiple multi-word synonyms, such as:

"poc" -> ["person of colour", "proof of concept"]

then search queries containing these synonyms will take much longer to resolve, especially if the queries are already long and contain many words with synonyms.

The reason for this slowdown mostly comes down to the fact that we treat a multi-word synonym not as a phrase but as a list of regular search query terms.

Therefore the following search query:

btw I am going to nyc soon

gets mapped to:

by the way I am going to New York City soon

as opposed to:

"by the way" I am going to "New York City" soon

It is worth considering the impact of treating multi-word synonyms as phrases in terms of relevancy. If the impact is neutral or just slightly negative, then we should make this change to ensure good search performance.

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TODO

• Implement changes in Milli
• Release a Milli version containing these changes
• Bump this new Milli version in Meilisearch and merge it into main

[curquiza]

Close by #3269, which integrates milli v0.38.0, which integrates the changes for this issue.

[curquiza]

@meilisearch/docs-team might be interested (it's up to you if you want to explain)