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Recently, the expression-rewrite effort changed the way that GROUP BY is implemented. Usually GROUP BY involves an aggregation function (e.g., if you want a separate SUM per partition). But there's also a query like SELECT p, c1, c2, v FROM tbl GROUP BY p This query is supposed to return one row - the *first* row in clustering order - per group (in this case, partition). The expression rewrite re-implemented this feature by introducing a new internal aggregator, first(), which returns the first aggregated value. The above query is rewritten into: SELECT first(p), first(c1), first(c2), first(v) FROM tbl GROUP BY p This case works correctly, and we even have a regression test for it. But unfortunately the rewrite broke the following query: SELECT * FROM tbl GROUP BY p Note the "*" instead of the explicit list of columns. In our implementation, a selection of "*" is looks like an empty selection, and it didn't get the "first()" treatment and it remained a "SELECT *" - and wrongly returned all rows instead of just the first one in each partition. This was a regression - it worked correctly in Scylla 5.2 (and also in Cassandra) - see the next patch for a regression test. In this patch we fix this regression. When there is a GROUP BY, the "*" is rewritten to the appropriate list of all visible columns and then gets the first() treatment, so it will return only the first row as expected. The next patch will be a test that confirms the bug and its fix. Fixes #16531 Signed-off-by: Nadav Har'El <nyh@scylladb.com>