mean() for decimals#8649
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Merging this PR will improve performance by 10.63%
Warning Please fix the performance issues or acknowledge them on CodSpeed. Performance Changes
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Mostly some clarifying questions
Signed-off-by: Mikhail Kot <mikhail@spiraldb.com>
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This PR adds support for pushing down ungropped aggregates min,max,sum,avg,mean,first,any_value,count(col), and count(*). It consists of roughly 3 parts: 1. Optimizer pass which works same as scalar function pushdown, but without visitors, because we can't express child extract with a visitor. 2. General accumulator handling in table_function.rs. This includes computing partials in every thread (so that we read data in parallel) and merging them, and also synchonization so that only one thread would write the global aggregated row count 3. Handling of count_star() (count(*)) a.k.a row count. Row count is a special accumulator because it doesn't need the array, and there's no column it aggregates. I've tried the implementation with CountStart being a separate accumulator, and it is much more complex than current one, because you can't provide a real column to accumulate on in select(), and you need to create a dummy one. So, we pay the cost of a separate atomic and a count_star positions vector, but our expression projection handling stays simple. We also reject count(*) if it's the only aggregate because duckdb calculating it natively (just using chunk lengths) is faster than our accumulator pipeline On the API input note, nearly all aggregate functions except for count_star() in duckdb have one non-const argument so input is a pair of a column and an expression. Depends on #8649 Signed-off-by: Mikhail Kot <mikhail@spiraldb.com>
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Support mean() for decimals (keeps mean as Decimal). Support casting decimals to f64 (we need this for aggregate pushdown because avg(decimal) in duckdb is f64).