When Brooklyn District Attorney Kenneth Thompson said this week that he wouldn't prosecute certain first-time offenders who possess small amounts of weed, he lit up a pungent debate among law enforcers.

More than a few fans of NYPD Commissioner William Bratton worry that Thompson's new policy undercuts the department's successful "broken-windows" theory -- which holds that tough enforcement of low-level crimes can lead to major reductions in more serious offenses.

But those qualms are over the top. While broken windows is a strong crime-fighting tool and a major reason for the city's crime drop, not all broken windows are equal.

A 2010 American Civil Liberties study found that Brooklyn cops arrested or gave summonses to 20,413 people that year for possessing small amounts of marijuana. Yet study after study has failed to establish a conclusive connection between small-time toking and societal ruin.

Meanwhile, weed patrol is costly. The ACLU calculates that all marijuana arrests in 2010 cost New York State taxpayers roughly $680 million in court and police costs.

And that's not the worst of it. Surveys show that weed enforcement missions -- in the city and elsewhere in the state -- are freighted with gross racial disparities.

Start with the fact that whites use grass at higher rates than blacks, according to research. Then note that in Brooklyn and Manhattan, black New Yorkers are nine times more likely to be busted for small-time possession of weed than whites. This gives countless kids police records that can sabotage their future.

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What Thompson has advanced is really a conservative plan. Only people not imbibing in public areas and not partaking around children will get a pass. But at least as DA, Thompson has the right to make these policies. Bratton isn't so lucky.

As top cop for all five boroughs, Bratton must continue with business as usual until Albany gives him new orders. A bill that would have done precisely that failed last year.

Albany must revive it and pass it next year.