ClickHouse Release 26.5

Summary

ClickHouse 26.5 is a monthly release containing 38 new features, 51 performance optimizations, and 224 bug fixes. It reports a record number of performance optimizations and introduces a filesystem table function for running SQL against a local file system. Its main query-planning examples push ORDER BY … LIMIT through eligible LEFT or RIGHT JOINs when the sort key depends only on the preserved side, allowing ClickHouse to limit rows before joining. In a TPC-H benchmark at scale factor 100, enabling this optimization reduced fastest runtime from 1.878 seconds to 0.092 seconds, peak memory from 1.88 GiB to 10.98 MiB, and data read by 1.5×. Additional changes include a GROUP BY … LIMIT shortcut for eligible unordered queries, an experimental in-browser clickhouse-client, and per-subquery query-cache controls.

Takeaways

1. 1

   ORDER BY … LIMIT pushdown is enabled by the query_plan_top_k_through_join setting and applies to LEFT or RIGHT JOIN queries when the sort key uses only columns from the preserved side.

2. 2

   In the stated TPC-H scale-factor-100 benchmark, the join pushdown reduced fastest runtime 20.4×, lowered peak memory usage by about 175×, and read 1.5× less data.

3. 3

   The experimental Web Terminal is enabled with allow_experimental_webterminal, while query caching can be enabled or disabled for individual subqueries or propagated into all subqueries.