We have tested it on a few queries with aggregation (with low cardinality results). For MySQL, fetching the raw data and doing aggregation in ClickHouse appeare

We have tested it on a few queries with aggregation (with low cardinality results). For MySQL, fetching the raw data and doing aggregation in ClickHouse appeared to be faster. For PostgreSQL it was identical (at least Postgres did not do significantly more work for aggregation than for data reading). It also depends on the network, but at least it was not limited by 10 Gbit network. Automatic pushdown of aggregations is currently not considered, but we consider a syntax to allow explicitly push down a whole subquery.