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This means that, for example, "SELECT expensive_func(a) FROM bigtab WHERE something" can compute expensive_func(a) in the workers rather than the leader if it happens to be parallel-safe, which figures to be a big win in some practical cases. Currently, we can only do this if the entire target list is parallel-safe. If we worked harder, we might be able to evaluate parallel-safe targets in the worker and any parallel-restricted targets in the leader, but that would be more complicated, and there aren't that many parallel-restricted functions that people are likely to use in queries anyway. I think. So just do the simple thing for the moment. Robert Haas, Amit Kapila, and Tom Lane
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