● Covered by 1 source · 1 reportLow impact1 positive
pg_clickhouse v0.10 Improves Subquery Pushdown, Speeds Up TPC-H Queries
Key points
- pg_clickhouse v0.10 released.
- 16 of 22 TPC-H queries now fully push down.
- Q17 performance improved by 1000x.
- Binary driver rebuilt on a new plain-C client library.
Subquery Pushdown Enhancements
pg_clickhouse version 0.10 has been released, focusing on improving subquery pushdown capabilities for analytical workloads. The update increases the number of TPC-H benchmark queries that can be fully pushed down to ClickHouse from 12 to 16 out of 22 total queries.
This advancement means that more complex queries are now processed directly by ClickHouse, reducing the need for PostgreSQL to fetch and evaluate data locally. The remaining six queries (Q13, Q15, Q16, Q18, Q20, Q21) are targeted for future pushdown improvements, with specific issues related to how PostgreSQL flattens subqueries into anti/semi-joins.
Performance Gains
The improved pushdown has led to significant performance increases for certain queries. For example, TPC-H Q17, a correlated subquery, now executes in 37 milliseconds when fully pushed down, compared to 32.7 seconds previously. This represents a 1000x speed improvement.
This particular query previously required pg_clickhouse to fetch every row from ClickHouse individually for local subquery evaluation, highlighting the efficiency gains from full pushdown.
Under-the-Hood Improvements
Beyond subquery pushdown, version 0.10 includes other technical updates. The binary driver has been rebuilt on a new plain-C client library. The surface area of functions and aggregates that support pushdown has more than doubled. Additionally, the binary driver has been hardened against several concurrency bugs.
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Primary sources
GitHub ClickHouse/ClickHouse GitHub ClickHouse/pg_clickhouse GitHub ClickHouse/clickhouse-cReporting from
pg_clickhouse released version 0.10, increasing the number of fully pushed-down TPC-H queries from 12 to 16 and improving performance for specific queries by up to 1000x. This update enhances the integration between PostgreSQL and ClickHouse for analytical workloads by reducing local processing and increasing the range of functions and aggregates that can be pushed down.