Merge #733

733: Avoid a prefix-related worst-case scenario in the proximity criterion r=loiclec a=loiclec

# Pull Request

## Related issue
Somewhat fixes (until merged into meilisearch) https://github.com/meilisearch/meilisearch/issues/3118

## What does this PR do?
When a query ends with a word and a prefix, such as:
```
word pr
```
Then we first determine whether `pre` *could possibly* be in the proximity prefix database before querying it. There are then three possibilities:

1. `pr` is not in any prefix cache because it is not the prefix of many words. We don't query the proximity prefix database. Instead, we list all the word derivations of `pre` through the FST and query the regular proximity databases.

2. `pr` is in the prefix cache but cannot be found in the proximity prefix databases. **In this case, we partially disable the proximity ranking rule for the pair `word pre`.** This is done as follows:
   1. Only find the documents where `word` is in proximity to `pre` **exactly** (no derivations)
   2. Otherwise, assume that their proximity in all the documents in which they coexist is >= 8

3. `pr` is in the prefix cache and can be found in the proximity prefix databases. In this case we simply query the proximity prefix databases.

Note that if a prefix is longer than 2 bytes, then it cannot be in the proximity prefix databases. Also, proximities larger than 4 are not present in these databases either. Therefore, the impact on relevancy is:

1. For common prefixes of one or two letters: we no longer distinguish between proximities from 4 to 8
2. For common prefixes of more than two letters: we no longer distinguish between any proximities
3. For uncommon prefixes: nothing changes

Regarding (1), it means that these two documents would be considered equally relevant according to the proximity rule for the query `heard pr` (IF `pr` is the prefix of more than 200 words in the dataset):
```json
[
    { "text": "I heard there is a faster proximity criterion" },
    { "text": "I heard there is a faster but less relevant proximity criterion" }
]
```

Regarding (2), it means that two documents would be considered equally relevant according to the proximity rule for the query "faster pro":
```json
[
    { "text": "I heard there is a faster but less relevant proximity criterion" }
    { "text": "I heard there is a faster proximity criterion" },
]
```
But the following document would be considered more relevant than the two documents above:
```json
{ "text": "I heard there is a faster swimmer who is competing in the pro section of the competition " }
```

Note, however, that this change of behaviour only occurs when using the set-based version of the proximity criterion. In cases where there are fewer than 1000 candidate documents when the proximity criterion is called, this PR does not change anything. 

---

## Performance

I couldn't use the existing search benchmarks to measure the impact of the PR, but I did some manual tests with the `songs` benchmark dataset.   

```
1. 10x 'a': 
	- 640ms ⟹ 630ms                  = no significant difference
2. 10x 'b':
	- set-based: 4.47s ⟹ 7.42        = bad, ~2x regression
	- dynamic: 1s ⟹ 870 ms           = no significant difference
3. 'Someone I l':
	- set-based: 250ms ⟹ 12 ms       = very good, x20 speedup
	- dynamic: 21ms ⟹ 11 ms          = good, x2 speedup 
4. 'billie e':
	- set-based: 623ms ⟹ 2ms         = very good, x300 speedup 
	- dynamic: ~4ms ⟹ 4ms            = no difference
5. 'billie ei':
	- set-based: 57ms ⟹ 20ms         = good, ~2x speedup
	- dynamic: ~4ms ⟹ ~2ms.          = no significant difference
6. 'i am getting o' 
	- set-based: 300ms ⟹ 60ms        = very good, 5x speedup
	- dynamic: 30ms ⟹ 6ms            = very good, 5x speedup
7. 'prologue 1 a 1:
	- set-based: 3.36s ⟹ 120ms       = very good, 30x speedup
	- dynamic: 200ms ⟹ 30ms          = very good, 6x speedup
8. 'prologue 1 a 10':
	- set-based: 590ms ⟹ 18ms        = very good, 30x speedup 
	- dynamic: 82ms ⟹ 35ms           = good, ~2x speedup
```

Performance is often significantly better, but there is also one regression in the set-based implementation with the query `b b b b b b b b b b`.

Co-authored-by: Loïc Lecrenier <loic.lecrenier@me.com>

milli/src/search/criteria/mod.rs

@@ -17,6 +17,7 @@ use super::query_tree::{Operation, PrimitiveQueryPart, Query, QueryKind};
 use super::CriterionImplementationStrategy;
 use crate::search::criteria::geo::Geo;
 use crate::search::{word_derivations, Distinct, WordDerivationsCache};
+use crate::update::{MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB, MAX_PROXIMITY_FOR_PREFIX_PROXIMITY_DB};
 use crate::{AscDesc as AscDescName, DocumentId, FieldId, Index, Member, Result};
 
 mod asc_desc;
@@ -653,14 +654,30 @@ fn query_pair_proximity_docids(
     match (&left.kind, &right.kind) {
         (QueryKind::Exact { word: left, .. }, QueryKind::Exact { word: right, .. }) => {
             if prefix {
-                match word_prefix_pair_overall_proximity_docids(
-                    ctx,
-                    left.as_str(),
-                    right.as_str(),
-                    proximity,
-                )? {
-                    Some(docids) => Ok(docids),
-                    None => {
+                // There are three distinct cases which we need to distinguish regarding the prefix `right`:
+                //
+                // 1. `right` is not in any prefix cache because it is not the prefix of many words
+                //     (and thus, it doesn't have many word derivations)
+                // 2. `right` is in the prefix cache but cannot be found in the "word prefix pair proximity" databases either
+                //     because it is too long or because the given proximity is too high.
+                // 3. `right` is in the prefix cache and can be found in the "word prefix pair proximity" databases
+                //
+                // The three cases are handled as follows:
+                // 1. We manually retrieve all the word derivations of `right` and check the `word_pair_proximity`
+                //    database for each of them.
+                // 2. It would be too expensive to apply the same strategy as (1), therefore, we "disable" the
+                //    proximity ranking rule for the prefixes of the right word. This is done as follows:
+                //    1. Only find the documents where left is in proximity to the exact (ie non-prefix) right word
+                //    2. Otherwise, assume that their proximity in all the documents in which they coexist is >= 8
+                //
+                // 3. Query the prefix proximity databases.
+                match (
+                    ctx.in_prefix_cache(right),
+                    right.len() <= MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB
+                        && proximity <= MAX_PROXIMITY_FOR_PREFIX_PROXIMITY_DB,
+                ) {
+                    // Case 1: not in prefix cache
+                    (false, _) => {
                         let r_words = word_derivations(right, true, 0, ctx.words_fst(), wdcache)?;
                         all_word_pair_overall_proximity_docids(
                             ctx,
@@ -669,10 +686,35 @@ fn query_pair_proximity_docids(
                             proximity,
                         )
                     }
+                    // Case 2: in prefix cache but either the prefix length or the proximity makes it impossible to
+                    // query the prefix proximity databases.
+                    (true, false) => {
+                        // To "save" the relevancy a little bit, we still find the documents where the
+                        // exact (i.e. non-prefix) right word is in the given proximity to the left word.
+                        Ok(word_pair_overall_proximity_docids(
+                            ctx,
+                            left.as_str(),
+                            right.as_str(),
+                            proximity,
+                        )?
+                        .unwrap_or_default())
+                    }
+                    // Case 3: in prefix cache, short enough, and proximity is low enough
+                    (true, true) => Ok(word_prefix_pair_overall_proximity_docids(
+                        ctx,
+                        left.as_str(),
+                        right.as_str(),
+                        proximity,
+                    )?
+                    .unwrap_or_default()),
                 }
             } else {
-                Ok(ctx
-                    .word_pair_proximity_docids(left.as_str(), right.as_str(), proximity)?
+                Ok(word_pair_overall_proximity_docids(
+                    ctx,
+                    left.as_str(),
+                    right.as_str(),
+                    proximity,
+                )?
                 .unwrap_or_default())
             }
         }
@@ -680,29 +722,55 @@ fn query_pair_proximity_docids(
             let l_words =
                 word_derivations(left, false, *typo, ctx.words_fst(), wdcache)?.to_owned();
             if prefix {
+                // The logic here is almost identical to the one in the previous match branch.
+                // The difference is that we fetch the docids for each derivation of the left word.
+                match (
+                    ctx.in_prefix_cache(right),
+                    right.len() <= MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB
+                        && proximity <= MAX_PROXIMITY_FOR_PREFIX_PROXIMITY_DB,
+                ) {
+                    // Case 1: not in prefix cache
+                    (false, _) => {
                         let mut docids = RoaringBitmap::new();
+                        let r_words = word_derivations(right, true, 0, ctx.words_fst(), wdcache)?;
                         for (left, _) in l_words {
-                    let current_docids = match word_prefix_pair_overall_proximity_docids(
-                        ctx,
-                        left.as_str(),
-                        right.as_str(),
-                        proximity,
-                    )? {
-                        Some(docids) => Ok(docids),
-                        None => {
-                            let r_words =
-                                word_derivations(right, true, 0, ctx.words_fst(), wdcache)?;
-                            all_word_pair_overall_proximity_docids(
+                            docids |= all_word_pair_overall_proximity_docids(
                                 ctx,
                                 &[(left, 0)],
                                 r_words,
                                 proximity,
-                            )
-                        }
-                    }?;
-                    docids |= current_docids;
+                            )?;
                         }
                         Ok(docids)
+                    }
+                    // Case 2: in prefix cache but either the prefix length or the proximity makes it impossible to
+                    // query the prefix proximity databases.
+                    (true, false) => {
+                        // To "save" the relevancy a little bit, we still find the documents where the
+                        // exact (i.e. non-prefix) right word is in proximity to any derivation of the left word.
+                        let mut candidates = RoaringBitmap::new();
+                        for (left, _) in l_words {
+                            candidates |= ctx
+                                .word_pair_proximity_docids(&left, right, proximity)?
+                                .unwrap_or_default();
+                        }
+                        Ok(candidates)
+                    }
+                    // Case 3: in prefix cache, short enough, and proximity is low enough
+                    (true, true) => {
+                        let mut docids = RoaringBitmap::new();
+                        for (left, _) in l_words {
+                            docids |= word_prefix_pair_overall_proximity_docids(
+                                ctx,
+                                left.as_str(),
+                                right.as_str(),
+                                proximity,
+                            )?
+                            .unwrap_or_default();
+                        }
+                        Ok(docids)
+                    }
+                }
             } else {
                 all_word_pair_overall_proximity_docids(ctx, &l_words, &[(right, 0)], proximity)
             }

milli/src/search/criteria/proximity.rs

@@ -590,3 +590,123 @@ fn resolve_plane_sweep_candidates(
 
     Ok(candidates)
 }
+
+#[cfg(test)]
+mod tests {
+    use std::io::Cursor;
+
+    use big_s::S;
+
+    use crate::documents::{DocumentsBatchBuilder, DocumentsBatchReader};
+    use crate::index::tests::TempIndex;
+    use crate::{CriterionImplementationStrategy, SearchResult};
+
+    fn documents_with_enough_different_words_for_prefixes(prefixes: &[&str]) -> Vec<crate::Object> {
+        let mut documents = Vec::new();
+        for prefix in prefixes {
+            for i in 0..500 {
+                documents.push(
+                    serde_json::json!({
+                        "text": format!("{prefix}{i:x}"),
+                    })
+                    .as_object()
+                    .unwrap()
+                    .clone(),
+                )
+            }
+        }
+        documents
+    }
+
+    #[test]
+    fn test_proximity_criterion_prefix_handling() {
+        let mut index = TempIndex::new();
+        index.index_documents_config.autogenerate_docids = true;
+
+        index
+            .update_settings(|settings| {
+                settings.set_primary_key(S("id"));
+                settings.set_criteria(vec![
+                    "words".to_owned(),
+                    "typo".to_owned(),
+                    "proximity".to_owned(),
+                ]);
+            })
+            .unwrap();
+
+        let mut documents = DocumentsBatchBuilder::new(Vec::new());
+
+        for doc in [
+            // 0
+            serde_json::json!({ "text": "zero is exactly the amount of configuration I want" }),
+            // 1
+            serde_json::json!({ "text": "zero bad configuration" }),
+            // 2
+            serde_json::json!({ "text": "zero configuration" }),
+            // 3
+            serde_json::json!({ "text": "zero config" }),
+            // 4
+            serde_json::json!({ "text": "zero conf" }),
+            // 5
+            serde_json::json!({ "text": "zero bad conf" }),
+        ] {
+            documents.append_json_object(doc.as_object().unwrap()).unwrap();
+        }
+        for doc in documents_with_enough_different_words_for_prefixes(&["conf"]) {
+            documents.append_json_object(&doc).unwrap();
+        }
+        let documents =
+            DocumentsBatchReader::from_reader(Cursor::new(documents.into_inner().unwrap()))
+                .unwrap();
+
+        index.add_documents(documents).unwrap();
+
+        let rtxn = index.read_txn().unwrap();
+
+        let SearchResult { matching_words: _, candidates: _, documents_ids } = index
+            .search(&rtxn)
+            .query("zero c")
+            .criterion_implementation_strategy(CriterionImplementationStrategy::OnlySetBased)
+            .execute()
+            .unwrap();
+        insta::assert_snapshot!(format!("{documents_ids:?}"), @"[2, 3, 4, 1, 5, 0]");
+
+        let SearchResult { matching_words: _, candidates: _, documents_ids } = index
+            .search(&rtxn)
+            .query("zero co")
+            .criterion_implementation_strategy(CriterionImplementationStrategy::OnlySetBased)
+            .execute()
+            .unwrap();
+        insta::assert_snapshot!(format!("{documents_ids:?}"), @"[2, 3, 4, 1, 5, 0]");
+
+        let SearchResult { matching_words: _, candidates: _, documents_ids } = index
+            .search(&rtxn)
+            .query("zero con")
+            .criterion_implementation_strategy(CriterionImplementationStrategy::OnlySetBased)
+            .execute()
+            .unwrap();
+        // Here searh results are degraded because `con` is in the prefix cache but it is too
+        // long to be stored in the prefix proximity databases, and we don't want to iterate over
+        // all of its word derivations
+        insta::assert_snapshot!(format!("{documents_ids:?}"), @"[0, 1, 2, 3, 4, 5]");
+
+        let SearchResult { matching_words: _, candidates: _, documents_ids } = index
+            .search(&rtxn)
+            .criterion_implementation_strategy(CriterionImplementationStrategy::OnlySetBased)
+            .query("zero conf")
+            .execute()
+            .unwrap();
+        // Here search results are degraded as well, but we can still rank correctly documents
+        // that contain `conf` exactly, and not as a prefix.
+        insta::assert_snapshot!(format!("{documents_ids:?}"), @"[4, 5, 0, 1, 2, 3]");
+
+        let SearchResult { matching_words: _, candidates: _, documents_ids } = index
+            .search(&rtxn)
+            .criterion_implementation_strategy(CriterionImplementationStrategy::OnlySetBased)
+            .query("zero config")
+            .execute()
+            .unwrap();
+        // `config` is not a common prefix, so the normal methods are used
+        insta::assert_snapshot!(format!("{documents_ids:?}"), @"[2, 3, 1, 0, 4, 5]");
+    }
+}

milli/src/update/mod.rs

@@ -7,7 +7,10 @@ pub use self::index_documents::{
     DocumentAdditionResult, DocumentId, IndexDocuments, IndexDocumentsConfig, IndexDocumentsMethod,
 };
 pub use self::indexer_config::IndexerConfig;
-pub use self::prefix_word_pairs::PrefixWordPairsProximityDocids;
+pub use self::prefix_word_pairs::{
+    PrefixWordPairsProximityDocids, MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB,
+    MAX_PROXIMITY_FOR_PREFIX_PROXIMITY_DB,
+};
 pub use self::settings::{Setting, Settings};
 pub use self::update_step::UpdateIndexingStep;
 pub use self::word_prefix_docids::WordPrefixDocids;

milli/src/update/prefix_word_pairs/mod.rs

@@ -14,6 +14,9 @@ mod word_prefix;
 pub use prefix_word::index_prefix_word_database;
 pub use word_prefix::index_word_prefix_database;
 
+pub const MAX_PROXIMITY_FOR_PREFIX_PROXIMITY_DB: u8 = 4;
+pub const MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB: usize = 2;
+
 pub struct PrefixWordPairsProximityDocids<'t, 'u, 'i> {
     wtxn: &'t mut heed::RwTxn<'i, 'u>,
     index: &'i Index,
@@ -32,31 +35,12 @@ impl<'t, 'u, 'i> PrefixWordPairsProximityDocids<'t, 'u, 'i> {
         Self {
             wtxn,
             index,
-            max_proximity: 4,
-            max_prefix_length: 2,
+            max_proximity: MAX_PROXIMITY_FOR_PREFIX_PROXIMITY_DB,
+            max_prefix_length: MAX_LENGTH_FOR_PREFIX_PROXIMITY_DB,
             chunk_compression_type,
             chunk_compression_level,
         }
     }
-    /// Set the maximum proximity required to make a prefix be part of the words prefixes
-    /// database. If two words are too far from the threshold the associated documents will
-    /// not be part of the prefix database.
-    ///
-    /// Default value is 4. This value must be lower or equal than 7 and will be clamped
-    /// to this bound otherwise.
-    pub fn max_proximity(&mut self, value: u8) -> &mut Self {
-        self.max_proximity = value.max(7);
-        self
-    }
-    /// Set the maximum length the prefix of a word pair is allowed to have to be part of the words
-    /// prefixes database. If the prefix length is higher than the threshold, the associated documents
-    /// will not be part of the prefix database.
-    ///
-    /// Default value is 2.
-    pub fn max_prefix_length(&mut self, value: usize) -> &mut Self {
-        self.max_prefix_length = value;
-        self
-    }
 
     #[logging_timer::time("WordPrefixPairProximityDocids::{}")]
     pub fn execute<'a>(