jiaqizho commented on PR #1187:
URL: https://github.com/apache/cloudberry/pull/1187#issuecomment-3004213098
> Just tried:
>
> ```
> -- 状态转换函数：处理单个输入值并更新状态
> CREATE OR REPLACE FUNCTION avg_trans_fn(
> state numeric[], -- 当前状态：[sum, count]
> next_val numeric -- 新输入值
> ) RETURNS numeric[] AS $$
> BEGIN
> IF next_val IS NOT NULL THEN
> state[1] := COALESCE(state[1], 0) + next_val;
> state[2] := COALESCE(state[2], 0) + 1;
> END IF;
> RETURN state;
> END;
> $$ LANGUAGE plpgsql;
>
> -- 合并函数：合并两个部分聚合状态
> CREATE OR REPLACE FUNCTION avg_combine_fn(
> state1 numeric[],
> state2 numeric[]
> ) RETURNS numeric[] AS $$
> BEGIN
> IF state1 IS NULL THEN
> RETURN state2;
> ELSIF state2 IS NULL THEN
> RETURN state1;
> ELSE
> RETURN ARRAY[state1[1] + state2[1], state1[2] + state2[2]];
> END IF;
> END;
> $$ LANGUAGE plpgsql;
>
> -- 最终函数：从状态计算最终结果
> CREATE OR REPLACE FUNCTION avg_final_fn(
> state numeric[]
> ) RETURNS numeric AS $$
> BEGIN
> IF state[2] = 0 THEN
> RETURN NULL;
> ELSE
> RETURN state[1] / state[2];
> END IF;
> END;
> $$ LANGUAGE plpgsql;
>
> CREATE AGGREGATE my_avg(numeric) (
> sfunc = avg_trans_fn, -- 状态转换函数
> stype = numeric[], -- 状态类型
> combinefunc = avg_combine_fn, -- 合并函数
> finalfunc = avg_final_fn, -- 最终计算函数
> initcond = '{0,0}' -- 初始状态
> );
>
> CREATE TABLE tbl1(v1 int, v2 numeric);
> CREATE TABLE tbl2(v1 int, v2 numeric);
> ```
>
> The agg pushdown doesn't work in ORCA but works in postgres Postgres query
optimizer.
Without any data in the table `tbl1` and `tbl2`. ORCA choose the one-stage
agg, but we do have the pushdown path.
we can use the `optimizer_force_multistage_agg` to force ORCA choose the
multi-stage path(But won't create the no-pushdown path, because the enforce of
distribution).
also use the `my_avg`
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