Unified search query syntax using the full-text search capabilities of the underlying DB. (#11635)

Support a unified search query syntax which leverages more of the full-text
search of each database supported by Synapse.

Supports, with the same syntax across Postgresql 11+ and Sqlite:

- quoted "search terms"
- `AND`, `OR`, `-` (negation) operators
- Matching words based on their stem, e.g. searches for "dog" matches
  documents containing "dogs". 

This is achieved by 

- If on postgresql 11+, pass the user input to `websearch_to_tsquery`
- If on sqlite, manually parse the query and transform it into the sqlite-specific
  query syntax.

Note that postgresql 10, which is close to end-of-life, falls back to using
`phraseto_tsquery`, which only supports a subset of the features.

Multiple terms separated by a space are implicitly ANDed.

Note that:

1. There is no escaping of full-text syntax that might be supported by the database;
  e.g. `NOT`, `NEAR`, `*` in sqlite. This runs the risk that people might discover this
  as accidental functionality and depend on something we don't guarantee.
2. English text is assumed for stemming. To support other languages, either the target
  language needs to be known at the time of indexing the message (via room metadata,
  or otherwise), or a separate index for each language supported could be created.

Sqlite docs: https://www.sqlite.org/fts3.html#full_text_index_queries
Postgres docs: https://www.postgresql.org/docs/11/textsearch-controls.html

1 files changed, 162 insertions, 35 deletions

diff --git a/synapse/storage/databases/main/search.py b/synapse/storage/databases/main/search.py
index 1b79acf955..a89fc54c2c 100644
--- a/synapse/storage/databases/main/search.py
+++ b/synapse/storage/databases/main/search.py
@@ -11,10 +11,22 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+import enum
 import logging
 import re
-from typing import TYPE_CHECKING, Any, Collection, Iterable, List, Optional, Set, Tuple
+from collections import deque
+from dataclasses import dataclass
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Collection,
+    Iterable,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    Union,
+)
 
 import attr
 
@@ -27,7 +39,7 @@ from synapse.storage.database import (
     LoggingTransaction,
 )
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
-from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
+from synapse.storage.engines import PostgresEngine, Sqlite3Engine
 from synapse.types import JsonDict
 
 if TYPE_CHECKING:
@@ -421,8 +433,6 @@ class SearchStore(SearchBackgroundUpdateStore):
         """
         clauses = []
 
-        search_query = _parse_query(self.database_engine, search_term)
-
         args: List[Any] = []
 
         # Make sure we don't explode because the person is in too many rooms.
@@ -444,20 +454,24 @@ class SearchStore(SearchBackgroundUpdateStore):
         count_clauses = clauses
 
         if isinstance(self.database_engine, PostgresEngine):
+            search_query = search_term
+            tsquery_func = self.database_engine.tsquery_func
             sql = (
-                "SELECT ts_rank_cd(vector, to_tsquery('english', ?)) AS rank,"
+                f"SELECT ts_rank_cd(vector, {tsquery_func}('english', ?)) AS rank,"
                 " room_id, event_id"
                 " FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?)"
+                f" WHERE vector @@  {tsquery_func}('english', ?)"
             )
             args = [search_query, search_query] + args
 
             count_sql = (
                 "SELECT room_id, count(*) as count FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?)"
+                f" WHERE vector @@ {tsquery_func}('english', ?)"
             )
             count_args = [search_query] + count_args
         elif isinstance(self.database_engine, Sqlite3Engine):
+            search_query = _parse_query_for_sqlite(search_term)
+
             sql = (
                 "SELECT rank(matchinfo(event_search)) as rank, room_id, event_id"
                 " FROM event_search"
@@ -469,7 +483,7 @@ class SearchStore(SearchBackgroundUpdateStore):
                 "SELECT room_id, count(*) as count FROM event_search"
                 " WHERE value MATCH ?"
             )
-            count_args = [search_term] + count_args
+            count_args = [search_query] + count_args
         else:
             # This should be unreachable.
             raise Exception("Unrecognized database engine")
@@ -501,7 +515,9 @@ class SearchStore(SearchBackgroundUpdateStore):
 
         highlights = None
         if isinstance(self.database_engine, PostgresEngine):
-            highlights = await self._find_highlights_in_postgres(search_query, events)
+            highlights = await self._find_highlights_in_postgres(
+                search_query, events, tsquery_func
+            )
 
         count_sql += " GROUP BY room_id"
 
@@ -510,7 +526,6 @@ class SearchStore(SearchBackgroundUpdateStore):
         )
 
         count = sum(row["count"] for row in count_results if row["room_id"] in room_ids)
-
         return {
             "results": [
                 {"event": event_map[r["event_id"]], "rank": r["rank"]}
@@ -542,9 +557,6 @@ class SearchStore(SearchBackgroundUpdateStore):
             Each match as a dictionary.
         """
         clauses = []
-
-        search_query = _parse_query(self.database_engine, search_term)
-
         args: List[Any] = []
 
         # Make sure we don't explode because the person is in too many rooms.
@@ -582,20 +594,23 @@ class SearchStore(SearchBackgroundUpdateStore):
             args.extend([origin_server_ts, origin_server_ts, stream])
 
         if isinstance(self.database_engine, PostgresEngine):
+            search_query = search_term
+            tsquery_func = self.database_engine.tsquery_func
             sql = (
-                "SELECT ts_rank_cd(vector, to_tsquery('english', ?)) as rank,"
+                f"SELECT ts_rank_cd(vector, {tsquery_func}('english', ?)) as rank,"
                 " origin_server_ts, stream_ordering, room_id, event_id"
                 " FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?) AND "
+                f" WHERE vector @@ {tsquery_func}('english', ?) AND "
             )
             args = [search_query, search_query] + args
 
             count_sql = (
                 "SELECT room_id, count(*) as count FROM event_search"
-                " WHERE vector @@ to_tsquery('english', ?) AND "
+                f" WHERE vector @@ {tsquery_func}('english', ?) AND "
             )
             count_args = [search_query] + count_args
         elif isinstance(self.database_engine, Sqlite3Engine):
+
             # We use CROSS JOIN here to ensure we use the right indexes.
             # https://sqlite.org/optoverview.html#crossjoin
             #
@@ -614,13 +629,14 @@ class SearchStore(SearchBackgroundUpdateStore):
                 " CROSS JOIN events USING (event_id)"
                 " WHERE "
             )
+            search_query = _parse_query_for_sqlite(search_term)
             args = [search_query] + args
 
             count_sql = (
                 "SELECT room_id, count(*) as count FROM event_search"
                 " WHERE value MATCH ? AND "
             )
-            count_args = [search_term] + count_args
+            count_args = [search_query] + count_args
         else:
             # This should be unreachable.
             raise Exception("Unrecognized database engine")
@@ -660,7 +676,9 @@ class SearchStore(SearchBackgroundUpdateStore):
 
         highlights = None
         if isinstance(self.database_engine, PostgresEngine):
-            highlights = await self._find_highlights_in_postgres(search_query, events)
+            highlights = await self._find_highlights_in_postgres(
+                search_query, events, tsquery_func
+            )
 
         count_sql += " GROUP BY room_id"
 
@@ -686,7 +704,7 @@ class SearchStore(SearchBackgroundUpdateStore):
         }
 
     async def _find_highlights_in_postgres(
-        self, search_query: str, events: List[EventBase]
+        self, search_query: str, events: List[EventBase], tsquery_func: str
     ) -> Set[str]:
         """Given a list of events and a search term, return a list of words
         that match from the content of the event.
@@ -697,6 +715,7 @@ class SearchStore(SearchBackgroundUpdateStore):
         Args:
             search_query
             events: A list of events
+            tsquery_func: The tsquery_* function to use when making queries
 
         Returns:
             A set of strings.
@@ -729,7 +748,7 @@ class SearchStore(SearchBackgroundUpdateStore):
                 while stop_sel in value:
                     stop_sel += ">"
 
-                query = "SELECT ts_headline(?, to_tsquery('english', ?), %s)" % (
+                query = f"SELECT ts_headline(?, {tsquery_func}('english', ?), %s)" % (
                     _to_postgres_options(
                         {
                             "StartSel": start_sel,
@@ -760,20 +779,128 @@ def _to_postgres_options(options_dict: JsonDict) -> str:
     return "'%s'" % (",".join("%s=%s" % (k, v) for k, v in options_dict.items()),)
 
 
-def _parse_query(database_engine: BaseDatabaseEngine, search_term: str) -> str:
-    """Takes a plain unicode string from the user and converts it into a form
-    that can be passed to database.
-    We use this so that we can add prefix matching, which isn't something
-    that is supported by default.
+@dataclass
+class Phrase:
+    phrase: List[str]
+
+
+class SearchToken(enum.Enum):
+    Not = enum.auto()
+    Or = enum.auto()
+    And = enum.auto()
+
+
+Token = Union[str, Phrase, SearchToken]
+TokenList = List[Token]
+
+
+def _is_stop_word(word: str) -> bool:
+    # TODO Pull these out of the dictionary:
+    #  https://github.com/postgres/postgres/blob/master/src/backend/snowball/stopwords/english.stop
+    return word in {"the", "a", "you", "me", "and", "but"}
+
+
+def _tokenize_query(query: str) -> TokenList:
+    """
+    Convert the user-supplied `query` into a TokenList, which can be translated into
+    some DB-specific syntax.
+
+    The following constructs are supported:
+
+    - phrase queries using "double quotes"
+    - case-insensitive `or` and `and` operators
+    - negation of a keyword via unary `-`
+    - unary hyphen to denote NOT e.g. 'include -exclude'
+
+    The following differs from websearch_to_tsquery:
+
+    - Stop words are not removed.
+    - Unclosed phrases are treated differently.
+
+    """
+    tokens: TokenList = []
+
+    # Find phrases.
+    in_phrase = False
+    parts = deque(query.split('"'))
+    for i, part in enumerate(parts):
+        # The contents inside double quotes is treated as a phrase, a trailing
+        # double quote is not implied.
+        in_phrase = bool(i % 2) and i != (len(parts) - 1)
+
+        # Pull out the individual words, discarding any non-word characters.
+        words = deque(re.findall(r"([\w\-]+)", part, re.UNICODE))
+
+        # Phrases have simplified handling of words.
+        if in_phrase:
+            # Skip stop words.
+            phrase = [word for word in words if not _is_stop_word(word)]
+
+            # Consecutive words are implicitly ANDed together.
+            if tokens and tokens[-1] not in (SearchToken.Not, SearchToken.Or):
+                tokens.append(SearchToken.And)
+
+            # Add the phrase.
+            tokens.append(Phrase(phrase))
+            continue
+
+        # Otherwise, not in a phrase.
+        while words:
+            word = words.popleft()
+
+            if word.startswith("-"):
+                tokens.append(SearchToken.Not)
+
+                # If there's more word, put it back to be processed again.
+                word = word[1:]
+                if word:
+                    words.appendleft(word)
+            elif word.lower() == "or":
+                tokens.append(SearchToken.Or)
+            else:
+                # Skip stop words.
+                if _is_stop_word(word):
+                    continue
+
+                # Consecutive words are implicitly ANDed together.
+                if tokens and tokens[-1] not in (SearchToken.Not, SearchToken.Or):
+                    tokens.append(SearchToken.And)
+
+                # Add the search term.
+                tokens.append(word)
+
+    return tokens
+
+
+def _tokens_to_sqlite_match_query(tokens: TokenList) -> str:
+    """
+    Convert the list of tokens to a string suitable for passing to sqlite's MATCH.
+    Assume sqlite was compiled with enhanced query syntax.
+
+    Ref: https://www.sqlite.org/fts3.html#full_text_index_queries
     """
+    match_query = []
+    for token in tokens:
+        if isinstance(token, str):
+            match_query.append(token)
+        elif isinstance(token, Phrase):
+            match_query.append('"' + " ".join(token.phrase) + '"')
+        elif token == SearchToken.Not:
+            # TODO: SQLite treats NOT as a *binary* operator. Hopefully a search
+            # term has already been added before this.
+            match_query.append(" NOT ")
+        elif token == SearchToken.Or:
+            match_query.append(" OR ")
+        elif token == SearchToken.And:
+            match_query.append(" AND ")
+        else:
+            raise ValueError(f"unknown token {token}")
+
+    return "".join(match_query)
 
-    # Pull out the individual words, discarding any non-word characters.
-    results = re.findall(r"([\w\-]+)", search_term, re.UNICODE)
 
-    if isinstance(database_engine, PostgresEngine):
-        return " & ".join(result + ":*" for result in results)
-    elif isinstance(database_engine, Sqlite3Engine):
-        return " & ".join(result + "*" for result in results)
-    else:
-        # This should be unreachable.
-        raise Exception("Unrecognized database engine")
+def _parse_query_for_sqlite(search_term: str) -> str:
+    """Takes a plain unicode string from the user and converts it into a form
+    that can be passed to sqllite's matchinfo().
+    """
+    return _tokens_to_sqlite_match_query(_tokenize_query(search_term))