db: index the seven unindexed FKs, drop the seven redundant ones (#3300, #3301)

A structural sweep of the deployed schema, run AFTER 0088 got the models
and the chain to exact agreement. That agreement is what 0088 achieved,
and it is worth naming what it does not prove: a models-vs-chain diff
shows the two describe the same schema, not that the schema is right.
Everything here was wrong in BOTH.

The one that matters: image_tag has PRIMARY KEY (image_record_id, tag_id)
and no other index, so tag_id is unindexed. That is the gallery's tag
filter (tag_query.py builds `image_tag.c.tag_id == tid`) and the ON
DELETE CASCADE from tag, both scanning the largest table in the schema.
Six more FKs were unindexed on smaller tables; presentation_review.tag_id
also CASCADEs.

Dropped, on the other side: ix_image_record_sha256 was an exact duplicate
of the index uq_image_record_sha256 already builds — two btrees on the
same column of the highest-insert-rate table. The other six are
single-column indexes a later composite superseded without the narrow one
being retired; a btree on (a,b) already serves lookups on a.

0088 deliberately taught the models to declare BOTH sha256 indexes so
they would describe reality. This changes the reality instead, and the
models change with it — otherwise the next baseline.yml run reintroduces
exactly the drift 0088 removed.

CONCURRENTLY throughout, so building the image_tag index does not hold an
ACCESS EXCLUSIVE lock over every write for the duration. The cost is that
the migration cannot run in a transaction and so is not atomic: every
statement is IF NOT EXISTS / IF EXISTS, making a re-run after a partial
failure safe. The docstring carries the query for finding an INVALID
index left by an interrupted CONCURRENTLY build.

What the sweep found clean, for the record: all 43 tables have a primary
key; all 51 FKs declare an explicit ON DELETE, so none silently blocks a
delete; the three enum CHECKs match the code that writes them (rule 36).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017QHszn9H8VBvx5Ke8x1hvw

alembic/versions/0089_index_hygiene.py

@@ -0,0 +1,120 @@
+"""Index the seven unindexed FKs; drop the seven redundant indexes (#3300, #3301).
+
+Found by a structural sweep of the deployed schema done AFTER 0088 brought the
+models and the migration chain into exact agreement. That agreement is what
+0088 achieved, and it is worth being precise about what it does NOT prove: a
+models-vs-chain diff shows the two describe the same schema. It says nothing
+about whether that schema is right. Everything here was wrong in BOTH, which is
+exactly the class of problem the reconciliation could not see.
+
+## Added: seven FK indexes
+
+`image_tag.tag_id` is the one that matters. The table's only index is
+PRIMARY KEY (image_record_id, tag_id), which leads with the wrong column for
+the two hottest things done with it:
+
+  * the gallery's tag filter — services/tag_query.py builds
+    `image_tag.c.tag_id == tid` (and `.in_(tids)`) on every tag-scoped browse;
+  * ON DELETE CASCADE from `tag` — deleting or merging a tag makes Postgres
+    find that tag's rows before it can remove them.
+
+Both had to scan the largest table in the schema. The other six are the same
+shape on much smaller tables; `presentation_review.tag_id` is the notable one,
+since it also CASCADEs.
+
+## Dropped: seven redundant indexes
+
+`ix_image_record_sha256` was an exact duplicate. A UNIQUE constraint builds its
+own index, so `uq_image_record_sha256` already covered the column and
+`image_record` carried two btrees on `sha256` — on the highest-insert-rate
+table in the system.
+
+The other six are single-column indexes that a later composite superseded
+without the narrow one being retired. A btree on (a, b) already serves lookups
+on `a`, so each was pure write amplification. `task_run` and `backup_run` are
+append-heavy operational logs, which is where that cost lands hardest.
+
+Note for anyone reading 0088 next to this: 0088 deliberately taught the models
+to declare BOTH sha256 indexes, so they would describe reality. That was right.
+This migration changes the reality instead, and the models change with it.
+
+## CONCURRENTLY, and why this migration has no transaction
+
+`CREATE INDEX` takes an ACCESS EXCLUSIVE lock for the whole build, which on
+`image_tag` means stalling every write for as long as it takes. CONCURRENTLY
+builds without blocking writers, at the cost of two table passes and an
+inability to run inside a transaction — hence `autocommit_block()`.
+
+The consequence to know about: this migration is NOT atomic. If it fails
+partway, the work already done stays done. Every statement is therefore written
+IF NOT EXISTS / IF EXISTS so that re-running it after a failure is safe rather
+than an error.
+
+A failed CONCURRENTLY build also leaves an INVALID index behind — it is not
+used by the planner and not repaired automatically. Find them with:
+
+    SELECT c.relname FROM pg_index i
+    JOIN pg_class c ON c.oid = i.indexrelid
+    WHERE NOT i.indisvalid;
+
+Drop what that returns and re-run; nothing else is needed.
+
+Revision ID: 0089
+Revises: 0088
+Create Date: 2026-08-31
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0089"
+down_revision: Union[str, None] = "0088"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+# (index name, table, column) — names match what the models render under
+# base.py's naming convention, so autogenerate stays quiet after this.
+MISSING_FK_INDEXES = (
+    ("ix_image_tag_tag_id", "image_tag", "tag_id"),
+    ("ix_presentation_review_tag_id", "presentation_review", "tag_id"),
+    ("ix_presentation_review_conflict_tag_id", "presentation_review", "conflict_tag_id"),
+    ("ix_import_task_result_image_id", "import_task", "result_image_id"),
+    ("ix_external_link_attachment_id", "external_link", "attachment_id"),
+    ("ix_character_prototype_region_id", "character_prototype", "region_id"),
+    ("ix_backup_run_restored_from_id", "backup_run", "restored_from_id"),
+)
+
+# (index name, table, column) — redundant; the second element of each pair in
+# the docstring is what still covers the column after the drop.
+REDUNDANT_INDEXES = (
+    ("ix_image_record_sha256", "image_record", "sha256"),
+    ("ix_backup_run_kind", "backup_run", "kind"),
+    ("ix_backup_run_status", "backup_run", "status"),
+    ("ix_task_run_queue", "task_run", "queue"),
+    ("ix_task_run_status", "task_run", "status"),
+    ("ix_task_run_task_name", "task_run", "task_name"),
+    ("ix_external_link_post_id", "external_link", "post_id"),
+)
+
+
+def upgrade() -> None:
+    with op.get_context().autocommit_block():
+        for name, table, column in MISSING_FK_INDEXES:
+            op.execute(
+                f"CREATE INDEX CONCURRENTLY IF NOT EXISTS {name} "
+                f"ON {table} ({column})"
+            )
+        for name, _table, _column in REDUNDANT_INDEXES:
+            op.execute(f"DROP INDEX CONCURRENTLY IF EXISTS {name}")
+
+
+def downgrade() -> None:
+    with op.get_context().autocommit_block():
+        for name, table, column in REDUNDANT_INDEXES:
+            op.execute(
+                f"CREATE INDEX CONCURRENTLY IF NOT EXISTS {name} "
+                f"ON {table} ({column})"
+            )
+        for name, _table, _column in MISSING_FK_INDEXES:
+            op.execute(f"DROP INDEX CONCURRENTLY IF EXISTS {name}")

backend/app/models/backup_run.py

@@ -37,9 +37,12 @@ class BackupRun(Base):
         Index("ix_backup_run_tag_partial", "tag", postgresql_where=text("tag IS NOT NULL")),
     )
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
-    kind: Mapped[str] = mapped_column(String(16), nullable=False, index=True)
+    # No index=True: ix_backup_run_kind_started (above) already leads with
+    # `kind`, so a single-column index on it was pure write cost (#3301).
+    kind: Mapped[str] = mapped_column(String(16), nullable=False)
     status: Mapped[str] = mapped_column(
-        String(16), nullable=False, default="pending", index=True,
+        # No index=True — ix_backup_run_status_finished leads with `status`.
+        String(16), nullable=False, default="pending",
         server_default="pending",
     )
     tag: Mapped[str | None] = mapped_column(String(64), nullable=True, index=True)
@@ -57,7 +60,9 @@ class BackupRun(Base):
     manifest: Mapped[dict] = mapped_column(
         JSON, nullable=False, default=dict, server_default="{}",
     )
+    # Self-referential FK, unindexed until 0089 (#3300): SET NULL has to find
+    # the rows pointing at a deleted run before it can null them.
     restored_from_id: Mapped[int | None] = mapped_column(
         ForeignKey("backup_run.id", ondelete="SET NULL"),
-        nullable=True,
+        nullable=True, index=True,
     )

backend/app/models/character_prototype.py

@@ -40,8 +40,10 @@ class CharacterPrototype(Base):
     )
     # Provenance: the region this vector was copied from. SET NULL so pruning a
     # region doesn't delete the prototype mid-cycle (the next refresh reconciles).
+    # index=True added in 0089 — the FK was unindexed (#3300).
     region_id: Mapped[int | None] = mapped_column(
-        ForeignKey("image_region.id", ondelete="SET NULL"), nullable=True
+        ForeignKey("image_region.id", ondelete="SET NULL"), nullable=True,
+        index=True,
     )
 
 

backend/app/models/external_link.py

@@ -57,11 +57,15 @@ class ExternalLink(Base):
         # — the same file linked twice in a post collapses to one row.
         Index("uq_external_link_post_url", "post_id", "url", unique=True),
         Index("ix_external_link_status", "status"),
+        # Unindexed FK (#3300).
+        Index("ix_external_link_attachment_id", "attachment_id"),
     )
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    # No index=True: uq_external_link_post_url (post_id, url) already leads
+    # with post_id (#3301).
     post_id: Mapped[int] = mapped_column(
-        ForeignKey("post.id", ondelete="CASCADE"), nullable=False, index=True
+        ForeignKey("post.id", ondelete="CASCADE"), nullable=False
     )
     artist_id: Mapped[int | None] = mapped_column(
         ForeignKey("artist.id", ondelete="SET NULL"), nullable=True, index=True

backend/app/models/image_record.py

@@ -59,9 +59,11 @@ class ImageRecord(Base):
 
     # On-disk identity
     path: Mapped[str] = mapped_column(Text, nullable=False, unique=True)
-    # index=True only: the UNIQUE half is the named constraint in
+    # Neither unique= nor index=: uq_image_record_sha256 in __table_args__
-    # __table_args__ above, matching what 0001 actually created.
+    # above creates its own index, and the separate ix_image_record_sha256
-    sha256: Mapped[str] = mapped_column(String(64), nullable=False, index=True)
+    # that 0001 also built was an exact duplicate of it — dropped in 0089
+    # (#3301). Lookups by sha256 use the constraint's index.
+    sha256: Mapped[str] = mapped_column(String(64), nullable=False)
     phash: Mapped[str | None] = mapped_column(String(32), nullable=True, index=True)
     size_bytes: Mapped[int] = mapped_column(BigInteger, nullable=False)
     mime: Mapped[str] = mapped_column(String(64), nullable=False)

backend/app/models/import_task.py

@@ -31,6 +31,8 @@ class ImportTask(Base):
 
     __table_args__ = (
         Index("ix_import_task_created_at_desc", text("created_at DESC")),
+        # Unindexed FK (#3300).
+        Index("ix_import_task_result_image_id", "result_image_id"),
     )
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
     batch_id: Mapped[int] = mapped_column(

backend/app/models/presentation_review.py

@@ -23,6 +23,10 @@ class PresentationReview(Base):
 
     __table_args__ = (
         Index("ix_presentation_review_resolved_at", "resolved_at"),
+        # Both FKs to tag were unindexed (#3300); tag_id CASCADEs, so a tag
+        # delete had to scan this table to find its rows.
+        Index("ix_presentation_review_tag_id", "tag_id"),
+        Index("ix_presentation_review_conflict_tag_id", "conflict_tag_id"),
     )
     image_record_id: Mapped[int] = mapped_column(
         ForeignKey("image_record.id", ondelete="CASCADE"), primary_key=True

backend/app/models/tag.py

@@ -71,6 +71,12 @@ image_tag = Table(
     Column("tag_id", ForeignKey("tag.id", ondelete="CASCADE"), primary_key=True),
     Column("source", String(32), nullable=False, default="manual", server_default="manual"),
     Column("created_at", DateTime(timezone=True), nullable=False, server_default=func.now()),
+    # The PK is (image_record_id, tag_id), which leads with the WRONG column
+    # for the two things that matter most here (#3300): the gallery's tag
+    # filter (tag_query.py builds `image_tag.c.tag_id == tid`) and the
+    # ON DELETE CASCADE from tag, which has to find a tag's rows to remove
+    # them. Without this index both scan the largest table in the schema.
+    Index("ix_image_tag_tag_id", "tag_id"),
 )
 
 

backend/app/models/task_run.py

@@ -35,8 +35,10 @@ class TaskRun(Base):
     celery_task_id: Mapped[str] = mapped_column(
         String(64), nullable=False, index=True,
     )
-    queue: Mapped[str] = mapped_column(String(32), nullable=False, index=True)
+    # Neither carries index=True: ix_task_run_queue_started and
-    task_name: Mapped[str] = mapped_column(String(128), nullable=False, index=True)
+    # ix_task_run_name_started already lead with these columns (#3301).
+    queue: Mapped[str] = mapped_column(String(32), nullable=False)
+    task_name: Mapped[str] = mapped_column(String(128), nullable=False)
     target_id: Mapped[int | None] = mapped_column(Integer, nullable=True)
     started_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, index=True,
@@ -46,7 +48,8 @@ class TaskRun(Base):
     )
     duration_ms: Mapped[int | None] = mapped_column(Integer, nullable=True)
     status: Mapped[str] = mapped_column(
-        String(16), nullable=False, default="running", index=True,
+        # No index=True — ix_task_run_status_started leads with `status`.
+        String(16), nullable=False, default="running",
         server_default="running",
     )
     error_type: Mapped[str | None] = mapped_column(String(128), nullable=True)