feat: pgvector hnsw index type (#19158)

* Adding hnsw index type for pgvector, allowing vector dimensions larger than 2000

* remove some variable assignments

* Make USE_HALFVEC variable configurable

* Simplify USE_HALFVEC handling

* Raise runtime error if the index requires rebuilt

---------

Co-authored-by: Moritz <moritz.mueller2@tu-dresden.de>

backend/open_webui/retrieval/vector/dbs/pgvector.py

@@ -1,4 +1,4 @@
-from typing import Optional, List, Dict, Any
+from typing import Optional, List, Dict, Any, Tuple
 import logging
 import json
 from sqlalchemy import (
@@ -22,7 +22,7 @@ from sqlalchemy.pool import NullPool, QueuePool
 
 from sqlalchemy.orm import declarative_base, scoped_session, sessionmaker
 from sqlalchemy.dialects.postgresql import JSONB, array
-from pgvector.sqlalchemy import Vector
+from pgvector.sqlalchemy import Vector, HALFVEC
 from sqlalchemy.ext.mutable import MutableDict
 from sqlalchemy.exc import NoSuchTableError
 
@@ -44,11 +44,20 @@ from open_webui.config import (
     PGVECTOR_POOL_MAX_OVERFLOW,
     PGVECTOR_POOL_TIMEOUT,
     PGVECTOR_POOL_RECYCLE,
+    PGVECTOR_INDEX_METHOD,
+    PGVECTOR_HNSW_M,
+    PGVECTOR_HNSW_EF_CONSTRUCTION,
+    PGVECTOR_IVFFLAT_LISTS,
+    PGVECTOR_USE_HALFVEC,
 )
 
 from open_webui.env import SRC_LOG_LEVELS
 
 VECTOR_LENGTH = PGVECTOR_INITIALIZE_MAX_VECTOR_LENGTH
+USE_HALFVEC = PGVECTOR_USE_HALFVEC
+
+VECTOR_TYPE_FACTORY = HALFVEC if USE_HALFVEC else Vector
+VECTOR_OPCLASS = "halfvec_cosine_ops" if USE_HALFVEC else "vector_cosine_ops"
 Base = declarative_base()
 
 log = logging.getLogger(__name__)
@@ -67,7 +76,7 @@ class DocumentChunk(Base):
     __tablename__ = "document_chunk"
 
     id = Column(Text, primary_key=True)
-    vector = Column(Vector(dim=VECTOR_LENGTH), nullable=True)
+    vector = Column(VECTOR_TYPE_FACTORY(dim=VECTOR_LENGTH), nullable=True)
     collection_name = Column(Text, nullable=False)
 
     if PGVECTOR_PGCRYPTO:
@@ -157,13 +166,9 @@ class PgvectorClient(VectorDBBase):
             connection = self.session.connection()
             Base.metadata.create_all(bind=connection)
 
-            # Create an index on the vector column if it doesn't exist
-            self.session.execute(
-                text(
-                    "CREATE INDEX IF NOT EXISTS idx_document_chunk_vector "
-                    "ON document_chunk USING ivfflat (vector vector_cosine_ops) WITH (lists = 100);"
-                )
-            )
+            index_method, index_options = self._vector_index_configuration()
+            self._ensure_vector_index(index_method, index_options)
+
             self.session.execute(
                 text(
                     "CREATE INDEX IF NOT EXISTS idx_document_chunk_collection_name "
@@ -177,6 +182,80 @@ class PgvectorClient(VectorDBBase):
             log.exception(f"Error during initialization: {e}")
             raise
 
+    @staticmethod
+    def _extract_index_method(index_def: Optional[str]) -> Optional[str]:
+        if not index_def:
+            return None
+        try:
+            after_using = index_def.lower().split("using ", 1)[1]
+            return after_using.split()[0]
+        except (IndexError, AttributeError):
+            return None
+
+    def _vector_index_configuration(self) -> Tuple[str, str]:
+        if PGVECTOR_INDEX_METHOD:
+            index_method = PGVECTOR_INDEX_METHOD
+            log.info(
+                "Using vector index method '%s' from PGVECTOR_INDEX_METHOD.",
+                index_method,
+            )
+        elif USE_HALFVEC:
+            index_method = "hnsw"
+            log.info(
+                "VECTOR_LENGTH=%s exceeds 2000; using halfvec column type with hnsw index.",
+                VECTOR_LENGTH,
+            )
+        else:
+            index_method = "ivfflat"
+
+        if index_method == "hnsw":
+            index_options = (
+                f"WITH (m = {PGVECTOR_HNSW_M}, ef_construction = {PGVECTOR_HNSW_EF_CONSTRUCTION})"
+            )
+        else:
+            index_options = f"WITH (lists = {PGVECTOR_IVFFLAT_LISTS})"
+
+        return index_method, index_options
+
+    def _ensure_vector_index(self, index_method: str, index_options: str) -> None:
+        index_name = "idx_document_chunk_vector"
+        existing_index_def = self.session.execute(
+            text(
+                """
+                SELECT indexdef
+                FROM pg_indexes
+                WHERE schemaname = current_schema()
+                  AND tablename = 'document_chunk'
+                  AND indexname = :index_name
+                """
+            ),
+            {"index_name": index_name},
+        ).scalar()
+
+        existing_method = self._extract_index_method(existing_index_def)
+        if existing_method and existing_method != index_method:
+            raise RuntimeError(
+                f"Existing pgvector index '{index_name}' uses method '{existing_method}' but configuration now "
+                f"requires '{index_method}'. Automatic rebuild is disabled to prevent long-running maintenance. "
+                "Drop the index manually (optionally after tuning maintenance_work_mem/max_parallel_maintenance_workers) "
+                "and recreate it with the new method before restarting Open WebUI."
+            )
+
+        if not existing_index_def:
+            index_sql = (
+                f"CREATE INDEX IF NOT EXISTS {index_name} "
+                f"ON document_chunk USING {index_method} (vector {VECTOR_OPCLASS})"
+            )
+            if index_options:
+                index_sql = f"{index_sql} {index_options}"
+            self.session.execute(text(index_sql))
+            log.info(
+                "Ensured vector index '%s' using %s%s.",
+                index_name,
+                index_method,
+                f" {index_options}" if index_options else "",
+            )
+
     def check_vector_length(self) -> None:
         """
         Check if the VECTOR_LENGTH matches the existing vector column dimension in the database.
@@ -196,16 +275,19 @@ class PgvectorClient(VectorDBBase):
         if "vector" in document_chunk_table.columns:
             vector_column = document_chunk_table.columns["vector"]
             vector_type = vector_column.type
-            if isinstance(vector_type, Vector):
-                db_vector_length = vector_type.dim
-                if db_vector_length != VECTOR_LENGTH:
-                    raise Exception(
-                        f"VECTOR_LENGTH {VECTOR_LENGTH} does not match existing vector column dimension {db_vector_length}. "
-                        "Cannot change vector size after initialization without migrating the data."
-                    )
-            else:
+            expected_type = HALFVEC if USE_HALFVEC else Vector
+
+            if not isinstance(vector_type, expected_type):
                 raise Exception(
-                    "The 'vector' column exists but is not of type 'Vector'."
+                    "The 'vector' column type does not match the expected type "
+                    f"('{expected_type.__name__}') for VECTOR_LENGTH {VECTOR_LENGTH}."
+                )
+
+            db_vector_length = getattr(vector_type, "dim", None)
+            if db_vector_length is not None and db_vector_length != VECTOR_LENGTH:
+                raise Exception(
+                    f"VECTOR_LENGTH {VECTOR_LENGTH} does not match existing vector column dimension {db_vector_length}. "
+                    "Cannot change vector size after initialization without migrating the data."
                 )
         else:
             raise Exception(
@@ -360,11 +442,11 @@ class PgvectorClient(VectorDBBase):
             num_queries = len(vectors)
 
             def vector_expr(vector):
-                return cast(array(vector), Vector(VECTOR_LENGTH))
+                return cast(array(vector), VECTOR_TYPE_FACTORY(VECTOR_LENGTH))
 
             # Create the values for query vectors
             qid_col = column("qid", Integer)
-            q_vector_col = column("q_vector", Vector(VECTOR_LENGTH))
+            q_vector_col = column("q_vector", VECTOR_TYPE_FACTORY(VECTOR_LENGTH))
             query_vectors = (
                 values(qid_col, q_vector_col)
                 .data(