[Feature Request]: Add read() support to Java ClickHouseIO (currently write-only)

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@nitinware is already working on this.

Since Sep 7, 2026.

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Description

io java new feature P2
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)

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