sql: improve query plan for find-running-job-of-type query

This commit improves the query plan for the `find-running-jobs-of-type`
query by adding a hint to use the `jobs_status_created_idx` and by
removing an `ORDER BY` clause if a job ID to ignore was not given. This
can eliminate an index join from the query plan in some cases, making
the query play more efficient.

Informs cockroachdb#107405

Release note: None

pkg/jobs/utils.go

@@ -58,15 +58,21 @@ func RunningJobExists(
  typeStrs = s.String()
  }
 
+ orderBy := " ORDER BY created"
+ if ignoreJobID == jobspb.InvalidJobID {
+ // There is no need to order by the created column if there is no job to
+ // ignore.
+ orderBy = ""
+ }
+
  stmt := `
 SELECT
  id
 FROM
- system.jobs
+ system.jobs@jobs_status_created_idx
 WHERE
  job_type IN ` + typeStrs + ` AND
- status IN ` + NonTerminalStatusTupleString + `
-ORDER BY created
+ status IN ` + NonTerminalStatusTupleString + orderBy + `
 LIMIT 1`
  it, err := txn.QueryIterator(
  ctx,