fix: add partial index for fast failure filtering on runs page (#8150)

When failures are sparse (<1%), filtering by failure status on the runs
page required scanning millions of success rows. Add a partial index on
v2_job_completed (workspace_id, completed_at DESC) WHERE status IN
('failure', 'canceled') and switch ORDER BY to completed_at when
filtering failures, so Postgres walks the small partial index directly.

Benchmarked at 5.2M rows / 1% failure rate:
- LIMIT 30:   800ms -> 0.4ms (2000x faster)
- LIMIT 1000: 550ms -> 21ms  (26x faster)

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

backend/migrations/20260228000000_v2_job_completed_failure_index.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS ix_v2_job_completed_failure_workspace;

backend/migrations/20260228000000_v2_job_completed_failure_index.up.sql

@@ -0,0 +1,7 @@
+-- Partial index for fast failure/canceled filtering on the runs page.
+-- When failures are sparse (<1%) this avoids scanning millions of successful jobs.
+-- The query orders by completed_at DESC (switched from created_at when success=false),
+-- so this index provides both filtering and ordering in a single scan.
+CREATE INDEX IF NOT EXISTS ix_v2_job_completed_failure_workspace
+    ON v2_job_completed (workspace_id, completed_at DESC)
+    WHERE status IN ('failure', 'canceled');

backend/windmill-api-jobs/src/query.rs

@@ -556,7 +556,10 @@ pub fn list_completed_jobs_query(
     let mut sqlb = SqlBuilder::select_from("v2_job_completed")
         .fields(fields)
         .order_by(
-            if lq.completed_before.is_some() || lq.completed_after.is_some() {
+            if lq.completed_before.is_some()
+                || lq.completed_after.is_some()
+                || lq.success == Some(false)
+            {
                 "v2_job_completed.completed_at"
             } else {
                 "v2_job.created_at"

backend/windmill-api/src/db.rs

@@ -81,6 +81,9 @@ lazy_static::lazy_static! {
                     (20260225100000, include_str!(
                         "../../migrations/20260225100000_asset_covering_index.up.sql"
                     ).replace("CREATE INDEX", "CREATE INDEX CONCURRENTLY").replace("DROP INDEX", "DROP INDEX CONCURRENTLY")),
+                    (20260228000000, include_str!(
+                        "../../migrations/20260228000000_v2_job_completed_failure_index.up.sql"
+                    ).replace("CREATE INDEX", "CREATE INDEX CONCURRENTLY")),
                     ].into_iter().collect();
 }