Fix: Use indexed stub_id for task query instead of external id (#754)

Resolve BE-2086

With these changes the query from the `listTaskWithRelatedQueryBuilder`
should come out looking like this:
```sql
SELECT
  t.*,
  w.external_id AS "workspace.external_id",
  w.name AS "workspace.name",
  s.external_id AS "stub.external_id",
  s.name AS "stub.name",
  s.config AS "stub.config",
  s.type AS "stub.type",
  d.external_id AS "deployment.external_id",
  d.name AS "deployment.name",
  d.version AS "deployment.version"
FROM
  task t
  JOIN workspace w ON t.workspace_id = w.id
  JOIN stub s ON t.stub_id = s.id
  LEFT JOIN deployment d ON s.id = d.stub_id
WHERE
  t.workspace_id = $1
  AND t.stub_id IN (SELECT id FROM stub WHERE external_id IN ($2))
```
instead of looking like this
```sql
SELECT
  t.*,
  w.external_id AS "workspace.external_id",
  w.name AS "workspace.name",
  s.external_id AS "stub.external_id",
  s.name AS "stub.name",
  s.config AS "stub.config",
  s.type AS "stub.type",
  d.external_id AS "deployment.external_id",
  d.name AS "deployment.name",
  d.version AS "deployment.version"
FROM
  task t
  JOIN workspace w ON t.workspace_id = w.id
  JOIN stub s ON t.stub_id = s.id
  LEFT JOIN deployment d ON s.id = d.stub_id
WHERE
  t.workspace_id = $1
  AND s.external_id IN ($2)
```
The difference is in how we are adding the filter for stub ids. The
combination of using a subquery to de-reference external ids into stub
ids and indexing the task table on stub id yields a large performance
improvement (13s -> ~0.02s on my machine).

pkg/repository/backend_postgres.go

@@ -548,7 +548,9 @@ func (c *PostgresBackendRepository) listTaskWithRelatedQueryBuilder(filters type
 	}
 
 	if len(filters.StubIds) > 0 {
-		qb = qb.Where(squirrel.Eq{"s.external_id": filters.StubIds})
+		// Subquery to get the stub ids from the external ids
+		stubIdsSubquery := squirrel.Select("id").From("stub").Where(squirrel.Eq{"external_id": filters.StubIds})
+		qb = qb.Where(squirrel.Expr("s.id in (?)", stubIdsSubquery))
 	}
 
 	if len(filters.StubNames) > 0 {

pkg/repository/backend_postgres_migrations/018_add_stubid_index_task.go

@@ -0,0 +1,22 @@
+package backend_postgres_migrations
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/pressly/goose/v3"
+)
+
+func init() {
+	goose.AddMigrationNoTxContext(upAddStubIdIndexTask, downAddStubIdIndexTask)
+}
+
+func upAddStubIdIndexTask(ctx context.Context, db *sql.DB) error {
+	_, err := db.ExecContext(ctx, `CREATE INDEX CONCURRENTLY IF NOT EXISTS task_stub_id on task(stub_id);`)
+	return err
+}
+
+func downAddStubIdIndexTask(ctx context.Context, db *sql.DB) error {
+	_, err := db.ExecContext(ctx, `DROP INDEX CONCURRENTLY IF EXISTS task_stub_id `)
+	return err
+}

pkg/repository/backend_postgres_test.go

@@ -0,0 +1,49 @@
+package repository
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"testing"
+
+	"github.com/beam-cloud/beta9/pkg/types"
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+	"github.com/stretchr/testify/require"
+)
+
+func TestListTaskWithRelated(t *testing.T) {
+	// Remove this skip if you are testing on local data
+	t.Skip()
+	ctx := context.Background()
+
+	dbName := "control_plane"
+	dbUser := "pguser"
+	connStr := fmt.Sprintf("postgres://%s@localhost:5433/%s?sslmode=disable", dbUser, dbName)
+	db, err := sql.Open("postgres", connStr)
+	require.NoError(t, err)
+	client := sqlx.NewDb(db, "postgres")
+	require.NotNil(t, client)
+	r := PostgresBackendRepository{
+		client: client,
+	}
+
+	workspaceId := uint(0)
+	stubId := uuid.New().String()
+	firstPageId := uint(0)
+	secondPageId := uint(0)
+
+	res, err := r.ListTasksWithRelatedPaginated(ctx, types.TaskFilter{WorkspaceID: workspaceId, StubIds: []string{stubId}})
+	require.NoError(t, err)
+	require.NotNil(t, res)
+	require.Equal(t, 10, len(res.Data))
+	require.Equal(t, firstPageId, res.Data[0].Id)
+	require.NotNil(t, res.Next)
+
+	res, err = r.ListTasksWithRelatedPaginated(ctx, types.TaskFilter{WorkspaceID: workspaceId, StubIds: []string{stubId}, Cursor: res.Next})
+	require.NoError(t, err)
+	require.NotNil(t, res)
+	require.Equal(t, 10, len(res.Data))
+	require.Equal(t, secondPageId, res.Data[0].Id)
+	require.NotNil(t, res.Next)
+}