Fix pg_trgm unavailability causing startup crash and silent retain failures (#626) (#649)

On managed PostgreSQL services (e.g. Azure Flexible Server), the pg_trgm
extension may not be available, causing two failures:

1. Migration c1a2b3d4e5f6 crashes on CREATE EXTENSION
2. Even if migration is bypassed, the default 'trigram' entity lookup
   strategy uses the % operator which requires pg_trgm, causing retain
   background tasks to fail silently

Changes:
- Migration now gracefully skips pg_trgm and index creation if the
  extension cannot be loaded
- EntityResolver auto-detects pg_trgm availability on first use and
  falls back to 'full' lookup strategy with a warning log

Co-authored-by: coder999999999 <coder999999999@users.noreply.github.com>

hindsight-api-slim/hindsight_api/alembic/versions/c1a2b3d4e5f6_enable_pg_trgm_and_entities_trgm_index.py

@@ -11,6 +11,7 @@ block; see migrations.py for how this is handled safely.
 
 from collections.abc import Sequence
 
+import sqlalchemy as sa
 from alembic import context, op
 
 revision: str = "c1a2b3d4e5f6"
@@ -25,9 +26,21 @@ def _get_schema_prefix() -> str:
 
 
 def upgrade() -> None:
-    # pg_trgm ships with every standard PostgreSQL installation as a contrib module.
+    # pg_trgm ships with most PostgreSQL installations as a contrib module.
     # It enables fast similarity lookups via GIN indexes, used for entity name matching.
-    op.execute("CREATE EXTENSION IF NOT EXISTS pg_trgm")
+    # On managed services (e.g. Azure Flexible Server), the extension may not be
+    # available or may require manual enablement.  We gracefully skip the index
+    # creation if the extension cannot be loaded — the entity resolver will
+    # auto-detect and fall back to the "full" lookup strategy at runtime.  See #626.
+    conn = op.get_bind()
+    try:
+        conn.execute(sa.text("CREATE EXTENSION IF NOT EXISTS pg_trgm"))
+    except Exception:
+        # Extension not available (managed Postgres, insufficient privileges, etc.)
+        # Roll back the failed statement and skip index creation.
+        conn.execute(sa.text("ROLLBACK"))
+        conn.execute(sa.text("BEGIN"))
+        return
 
     schema = _get_schema_prefix()
     # GIN index on canonical_name enables sub-millisecond trigram similarity queries

hindsight-api-slim/hindsight_api/engine/entity_resolver.py

@@ -75,6 +75,7 @@ class EntityResolver:
         """
         self.pool = pool
         self.entity_lookup = entity_lookup
+        self._pg_trgm_checked = False
         # Keyed by asyncio task id so concurrent retain batches never mix their
         # pending updates.  flush_pending_stats() pops only the calling task's items.
         self._pending_stats: dict[int, list[_EntityStat]] = {}
@@ -202,6 +203,24 @@ class EntityResolver:
         taxonomy_lookup: set[str] | None = None,
     ) -> list[str]:
         if self.entity_lookup == "trigram":
+            # Auto-detect pg_trgm availability on first call and fall back to
+            # "full" strategy if the extension is not installed.  See #626.
+            if not self._pg_trgm_checked:
+                self._pg_trgm_checked = True
+                has_trgm = await conn.fetchval(
+                    "SELECT EXISTS(SELECT 1 FROM pg_extension WHERE extname = 'pg_trgm')"
+                )
+                if not has_trgm:
+                    logger.warning(
+                        "pg_trgm extension is not available — falling back to 'full' "
+                        "entity lookup strategy. Install pg_trgm for faster entity "
+                        "resolution on large banks. See: "
+                        "https://github.com/vectorize-io/hindsight/issues/626"
+                    )
+                    self.entity_lookup = "full"
+                    return await self._resolve_entities_batch_full(
+                        conn, bank_id, entities_data, unit_event_date
+                    )
             return await self._resolve_entities_batch_trigram(conn, bank_id, entities_data, unit_event_date)
         return await self._resolve_entities_batch_full(conn, bank_id, entities_data, unit_event_date)