[r2dbc] getRowsUpdated() returns 0 for successful INSERT…SELECT — needs reliable written_rows

从 ClickHouseResult 开始，跟踪 ClickHouseResponseSummary.getProgress()、getStatistics() 和顶层的 getWrittenRows() 如何提供 UpdateCount。使用 HTTP transport 重现一条 INSERT INTO … SELECT 查询，然后将 getRowsUpdated() 与最终的 written_rows 值进行比较。当选定的完成后权威值能够可靠地公开，并且有针对成功 INSERT…SELECT 的回归覆盖时，即表示完成。

Summary

ClickHouseResult.getRowsUpdated() in clickhouse-r2dbc returns 0 for
successful INSERT INTO … SELECT FROM … queries that did write rows. This
makes it impossible to reliably distinguish "INSERT…SELECT wrote 0 rows
because the SELECT produced none" from "INSERT…SELECT wrote N rows but the
driver reported 0" at the application layer.

The same row count appears correctly in system.query_log.written_rows
server-side, and the HTTP X-ClickHouse-Summary header also carries an
accurate written_rows once the query finishes. The information exists; the
driver just doesn't expose it via the standard R2DBC Result.getRowsUpdated()
contract for this query shape.

Reproduction

• Driver: com.clickhouse:clickhouse-r2dbc:0.9.0 (also reproduces on 0.8.x)
• Server: ClickHouse 25.3
• Query shape: INSERT INTO target_table (...) SELECT ... FROM source_table WHERE ...
• Connection settings: async_insert=1, wait_for_async_insert=1
  (per docs, async_insert is a no-op for INSERT…SELECT, but we set it
  globally for the INSERT VALUES path on the same connection)

Flux.from(statement.execute())
    .flatMap(Result::getRowsUpdated)   // emits 0 even when N rows were inserted
    .reduce(0L, Long::sum)
    // observed: returns 0

Verifying server-side after the query finishes:

SELECT written_rows
FROM system.query_log
WHERE query_id = '...' AND type = 'QueryFinish';
-- returns N (the correct count)

Root cause

Looking at ClickHouseResult constructor (current main):

Mono<? extends UpdateCount> updatedCount = Mono.just(response)
    .map(ClickHouseResponse::getSummary)
    .map(ClickHouseResponseSummary::getProgress)
    .map(ClickHouseResponseSummary.Progress::getWrittenRows)
    .map(UpdateCount::new);

The driver reads written_rows from Summary.getProgress(), which is the
interim progress event snapshot — not the final summary. For
INSERT…SELECT queries, a definitive post-completion written_rows is
typically reflected in Summary.getStatistics() (or in a final progress
event that doesn't always land before the subscriber observes completion).

ClickHouseResponseSummary exposes both getProgress() and getStatistics(),
and the top-level getWrittenRows() delegates to progress.

Use case

Detecting at the application layer when an INSERT…SELECT wrote zero rows
(to surface inconsistency conditions before committing dependent state).
Since getRowsUpdated() can return 0 even on success, the check fires
false positives.

Asks

Any one of the following would unblock us:

1. Source getRowsUpdated() from Summary.getStatistics() (or whichever
   field is authoritative post-completion) instead of from Summary.getProgress().
2. Expose the raw ClickHouseResponseSummary from ClickHouseResult (or a
   similar handle), so callers can read the final fields themselves.
3. Document the current semantics so applications know not to rely on
   getRowsUpdated() for INSERT…SELECT.

Happy to send a PR for (1) or (2) — please confirm which direction you'd
prefer.

Environment

• clickhouse-r2dbc: 0.9.0
• clickhouse-client / clickhouse-http-client: 0.9.0
• ClickHouse server: 25.3.x
• Connection: HTTP transport