Pushdown percentile_cont() as quantile()#36
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Detect use of `percentile_cont()`, an ordered set aggregate, and push it down to ClickHouse as `quantile()`. Like `quantile()`'s parameters, the argument to `percentile_cont()` is evaluates only once, but the arguments to `WITHIN GROUP (ORDER BY)` are evaluated for each row. In examples seen in the wild, including [HouseClick] and [this blog post], this appears to be a valid mapping, as long as there is no `DESC`, `USING >`, or `NULLS FIRST`. In the future we should be able to map `percentile_cont(float[])` to `quantiles()`, though it will require converting the array argument to a parameter list. It might also make sense to map `percentile_disc` to `quantileExactHigh()`. To implement this syntax conversion, `deparseAggref()` borrows from `postgres_fdw` to output the arguments to the aggregate as a parameter list and then the `WITHIN GROUP (ORDER)` as the normal parameter arguments. Thus this PostgreSQL query: ```sql SELECT percentile_cont(0.25) WITHIN GROUP (ORDER BY a) FROM t1; ``` Maps to this ClickHouse query: ```sql SELECT quantile(0.25)(a) FROM t1; ``` The non-default `ORDER BY` suffixes `DESC` and `NULLS FIRST` are not supported and will raise an error. For normal aggregates it keeps the previous behavior. [HouseClick]: https://github.com/ClickHouse/HouseClick/blob/fa449b2/app/lib/analytics_queries.ts [this blog post]: https://clickhouse.com/blog/redshift-vs-clickhouse-comparison#ethereum-gas-used-by-week "Optimizing Analytical Workloads: Comparing Redshift vs ClickHouse"
serprex
approved these changes
Nov 11, 2025
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Detect use of
percentile_cont(), an ordered set aggregate, and push it down to ClickHouse asquantile(). Likequantile()'s parameters, the argument topercentile_cont()is evaluates only once, but the arguments toWITHIN GROUP (ORDER BY)are evaluated for each row.In examples seen in the wild, including HouseClick and this blog post, this appears to be a valid mapping, as long as there is no
DESC,USING >, orNULLS FIRST.In the future we should be able to map
percentile_cont(float[])toquantiles(), though it will require converting the array argument to a parameter list. It might also make sense to mappercentile_disctoquantileExactHigh().To implement this syntax conversion,
deparseAggref()borrows frompostgres_fdwto output the arguments to the aggregate as a parameter list and then theWITHIN GROUP (ORDER)as the normal parameter arguments. Thus this PostgreSQL query:Maps to this ClickHouse query:
The non-default
ORDER BYsuffixesDESCandNULLS FIRSTare not supported and will raise an error.For normal aggregates it keeps the previous behavior.