**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

airflow/jobs/scheduler_job.py
pool_slots_free = max(0, sum(pool['open'] for pool in pools.values()))
        if pool_slots_free == 0:
            self.log.debug("All pools are full!")
            return executable_tis
        max_tis = min(max_tis, pool_slots_free)
        
        
        
        query = (
            session.query(TI)
            .outerjoin(TI.dag_run)
            .filter(or_(DR.run_id.is_(None), DR.run_type != DagRunType.BACKFILL_JOB))
            .join(TI.dag_model)
            .filter(not_(DM.is_paused))
            .filter(TI.state == State.SCHEDULED)
            .options(selectinload('dag_model'))
            .limit(max_tis)
        )
        task_instances_to_examine: List[TI] = with_row_locks(
            query,
            of=TI,
            session=session,
            **skip_locked(session=session),
        ).all()
        
        
        if len(task_instances_to_examine) == 0:
            self.log.debug("No tasks to consider for execution.")