perf: index the FK columns that cascade on workspace delete (#11052)

Claude-Session: https://claude.ai/code/session_01UtSL61CGQqtLc28AT2h75d

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/migrations/20260909163047_workspace_delete_cascade_indexes.down.sql

@@ -0,0 +1,7 @@
+DROP INDEX IF EXISTS index_app_version_on_app_id;
+
+DROP INDEX IF EXISTS index_app_script_on_app;
+
+DROP INDEX IF EXISTS index_workspace_runnable_dependencies_on_app_path;
+
+DROP INDEX IF EXISTS index_workspace_runnable_dependencies_on_flow_path;

backend/migrations/20260909163047_workspace_delete_cascade_indexes.up.sql

@@ -0,0 +1,32 @@
+-- The FK columns that cascade when a workspace's apps and flows are deleted. Unindexed,
+-- Postgres seq-scans the whole child table once per deleted parent row, making a workspace
+-- delete cost O(apps and flows deleted x rows in the instance). Fork deletion is where that
+-- bites: a fork clones its parent's apps, flows and entire app version history.
+--
+-- The workspace_runnable_dependencies pair are partial because the table's check constraint
+-- makes app_path and flow_path mutually exclusive, halving each index -- the cascade's
+-- equality on the path proves the predicate. The table's existing path indexes are partial on
+-- script_hash, which the cascade does not constrain, so they cannot serve it.
+--
+-- Dropped before built: a failed concurrent build leaves an invalid index that IF NOT EXISTS
+-- would accept forever, unused by the planner yet still maintained on every write.
+--
+-- No statement separators outside the statements below, comments included: the CONCURRENTLY
+-- rewrite in windmill-api/src/db.rs splits the file on them and would run comment text as SQL.
+DROP INDEX IF EXISTS index_app_version_on_app_id;
+
+CREATE INDEX index_app_version_on_app_id ON app_version (app_id);
+
+DROP INDEX IF EXISTS index_app_script_on_app;
+
+CREATE INDEX index_app_script_on_app ON app_script (app);
+
+DROP INDEX IF EXISTS index_workspace_runnable_dependencies_on_app_path;
+
+CREATE INDEX index_workspace_runnable_dependencies_on_app_path
+    ON workspace_runnable_dependencies (app_path, workspace_id) WHERE app_path IS NOT NULL;
+
+DROP INDEX IF EXISTS index_workspace_runnable_dependencies_on_flow_path;
+
+CREATE INDEX index_workspace_runnable_dependencies_on_flow_path
+    ON workspace_runnable_dependencies (flow_path, workspace_id) WHERE flow_path IS NOT NULL;

backend/windmill-api/src/db.rs

@@ -108,6 +108,9 @@ lazy_static::lazy_static! {
                     (20260826214706, include_str!(
                         "../../migrations/20260826214706_queue_suspended_drop_legacy_index.up.sql"
                     ).replace("DROP INDEX", "DROP INDEX CONCURRENTLY")),
+                    (20260909163047, include_str!(
+                        "../../migrations/20260909163047_workspace_delete_cascade_indexes.up.sql"
+                    ).replace("CREATE INDEX", "CREATE INDEX CONCURRENTLY").replace("DROP INDEX", "DROP INDEX CONCURRENTLY")),
                     ].into_iter().collect();
 }