nitinware opened a new issue, #40048:
URL: https://github.com/apache/beam/issues/40048
### What would you like to happen?
## Motivation
The Java `ClickHouseIO` connector at `sdks/java/io/clickhouse/` currently
supports **only writes**. Reading from ClickHouse in a Beam pipeline requires
falling back to `JdbcIO` with the ClickHouse JDBC driver, which loses
ClickHouse-specific schema fidelity (`LowCardinality`, `Array(T)`,
`Nullable(T)`, `Nested`, tuples) and native columnar transport.
## Proposal
Add `ClickHouseIO.read()` and `ClickHouseIO.readAll()` PTransforms symmetric
to the existing write path:
- Native `Row` output using the ClickHouse client's native format
- Query/projection via user-supplied `SELECT` template
- Splittable reader for parallel reads across partitions / shards,
modeled on `JdbcIO`'s splitting pattern
- Reuse the existing connection configuration builder from the writer
## Benefits
- Closes an obvious read/write asymmetry — every other major DB IO in
Beam (JDBC, Cassandra, Mongo, HBase, Spanner, BigQuery, Snowflake)
supports both directions
- ClickHouse is increasingly common as an analytics source; users
currently work around this with `JdbcIO` at the cost of type fidelity
## Testing plan
- Unit tests mirroring the existing writer test file
- Integration test using the module's existing testcontainers-based
ClickHouse setup (`ClickHouseContainer` is already wired in `build.gradle`)
- No new dependencies required
### Issue Priority
Priority: 2 (default / most feature requests should be filed as P2)
### Issue Components
- [ ] Component: Python SDK
- [x] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [x] Component: IO connector
- [ ] Component: Beam YAML
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Infrastructure
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Prism Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner
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