ClickHouseDatasetQueryBackend

The four query primitives over one dataset's table. Every statement runs under the limits of the query's entry (ClickHouseDatasetOptions.limits) and reads FINAL when the dataset deduplicates.

A record carries the dataset's declared fields only. A keyset page orders by the cursor sort, whose fields are non-Nullable columns (the only ones granted cursor sort), and reads each row's position from extra columns: a date as epoch milliseconds, a decimal as its exact text, anything else as its JSON value.