fix: optimize slow list_assets query for recents loading (#8103)

* fix: optimize slow list_assets query with covering index and v2_job join fix

Add a covering index on asset(workspace_id, path, kind, created_at DESC, id DESC)
with INCLUDE(usage_kind, usage_path) to enable index-only scans for the CTE aggregation.
Fix v2_job join to cast asset.usage_path::uuid instead of job.id::text, allowing
PostgreSQL to use the job_pkey primary key index instead of seq scanning the entire table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: drop redundant asset indexes subsumed by new covering index

idx_asset_workspace_created_id and idx_asset_kind_path are fully covered
by the new idx_asset_ws_path_kind_recent + the primary key. Verified all
asset table queries still have optimal index coverage. Reduces write
amplification on inserts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/migrations/20260225100000_asset_covering_index.down.sql

@@ -0,0 +1,5 @@
+DROP INDEX IF EXISTS idx_asset_ws_path_kind_recent;
+
+-- Restore the dropped indexes
+CREATE INDEX IF NOT EXISTS idx_asset_workspace_created_id ON asset (workspace_id, created_at DESC, id DESC);
+CREATE INDEX IF NOT EXISTS idx_asset_kind_path ON asset (workspace_id, kind, path);

backend/migrations/20260225100000_asset_covering_index.up.sql

@@ -0,0 +1,11 @@
+-- Covering index for the list_assets CTE: GROUP BY (path, kind) + MAX(created_at, id) + ORDER BY
+-- Includes usage_kind and usage_path to allow full index-only scan (avoiding heap lookups for filter conditions)
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_asset_ws_path_kind_recent
+    ON asset (workspace_id, path, kind, created_at DESC, id DESC)
+    INCLUDE (usage_kind, usage_path);
+
+-- Drop indexes now subsumed by idx_asset_ws_path_kind_recent:
+-- idx_asset_workspace_created_id (workspace_id, created_at DESC, id DESC) - only used by list_assets CTE
+-- idx_asset_kind_path (workspace_id, kind, path) - only used by list_assets CTE/outer join, covered by new index + PK
+DROP INDEX CONCURRENTLY IF EXISTS idx_asset_workspace_created_id;
+DROP INDEX CONCURRENTLY IF EXISTS idx_asset_kind_path;

backend/windmill-api-assets/src/lib.rs

@@ -144,7 +144,7 @@ async fn list_assets(
         format!(
             r#"FROM asset
             LEFT JOIN v2_job job_cte ON asset.usage_kind = 'job'
-              AND asset.usage_path = job_cte.id::text
+              AND job_cte.id = CASE WHEN asset.usage_kind = 'job' THEN asset.usage_path::uuid END
               AND job_cte.workspace_id = $1"#
         )
     } else {
@@ -209,7 +209,7 @@ async fn list_assets(
           ) = resource.path
           AND resource.workspace_id = $1
         LEFT JOIN v2_job job ON asset.usage_kind = 'job'
-          AND asset.usage_path = job.id::text
+          AND job.id = CASE WHEN asset.usage_kind = 'job' THEN asset.usage_path::uuid END
           AND job.workspace_id = $1
         WHERE asset.workspace_id = $1
           AND (asset.kind <> 'resource' OR resource.path IS NOT NULL)