User-defined aggregate functions have been supported in IRIS since 2021.1.0. I'd wished upon a star for these years ago before finding a secret hacky way to override MAX and MIN in a custom datatype, but didn't get a chance to actually try one out until today. I thought it was an interesting experience/example - the question of how to get a Median in IRIS SQL came up once before - so I'm sharing it here without too much further comment.
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