In a traffic case, if it’s set for trial, the court issues a notice to appear for the officer. If they’re properly served and don’t show on the trial date, case dismissed. Not so in a DUI, however.
There are several stages of a misdemeanor case such as a DUI. The first court appearance is the arraignment. You’re officially told of the charges against you in a charging document filed by the prosecution. Your attorney is also provided the police reports or other “discovery” (blood alcohol results, etc.). The officer is not required to be present at the arraignment.
If your case does not resolve by way of a motion to dismiss, plea deal or other outcome and is set for trial, the prosecution will then send out subpoenas for the witnesses they feel are necessary for their case. The officers are usually subpoenaed, but placed “on call” so they’re brought in only when necessary.
If the prosecutor cannot confirm the subpoena has been served on the officer, then there are certain ways for them to buy some more time.
In California, a defendant on a misdemeanor has a right to a trial within 30 days of their arraignment if they’re in custody at the time of the arraignment and within 45 days if they’re out of custody. If they waive that speedy trial right, then their case gets set on a particular date, but there is a 10-day trailing period. The day the case is calendared for trial is “day zero”. If for some reason, the prosecution is not ready, they can request to trail within this 10 day window while they get ready. This often happens with an officer not being available on the first day or two of when a trial is set due to vacation or training schedules, but they’ll be back by day 3. The prosecution will ask to trail the case to day 3 and then announce they’re ready for trial. The defendant’s rights are still protected and the case goes forward.
Now - if the officer is unavailable for some reason and the time frame is going to expire, the prosecution may make a motion to continue beyond that time frame based on “good cause”. It can’t be because of negligence like forgetting to subpoena the officer, but for a compelling reason, a judge can continue a case beyond the time frames. For example, if the officer that’s crucial to the prosecutor on the DUI case was injured on duty a week before the trial was scheduled to begin and unable to come testify for a couple of weeks, the prosecution would make a motion to continue the case until the officer is able to return to duty and testify. Such a motion would likely be granted, even over the defendant’s objection. If the case is continued over the defendant’s objection, then the new trial date is THE trial date - no more additional 10 day grace period. Either the case gets underway, it gets continued for a new “good cause” motion or it gets dismissed.
If the prosecution is unable to produce the officer at trial, has run out of any trailing period and their request to continue is not granted, then yes - without the testimony of the officer that made the arrest, the case would be dismissed.
It’s just not as straight-forward as a traffic ticket trial.