ClickHouse release 26.8

Summary

ClickHouse 26.8 is a long-term support release with 98 new features, 128 performance optimizations, and 556 bug fixes. It adds background queries, PostgreSQL-style regular-expression operators, array subscripting with multiple indices, expanded Parquet and data-lake integrations, and improvements for GROUP BY and joins. Background queries return immediately while continuing after client disconnection, with progress exposed through system.processes and completion details recorded in system.query_log; the example exports 243,619,704 rows to Parquet. Performance work includes dictionary-filter pushdown for Parquet, adaptive parallel GROUP BY processing, smaller hash-table cells for single-string keys, parallel merge joins, IEJoin for two-inequality conditions, and a cost-based optimizer for distributed plans. The PostgreSQL wire protocol supports psql commands such as \d, \dt, and \dv while the release adds text tokenizers.

Takeaways

1. 1

   Setting run_query_in_background to 1 lets a query return immediately and continue after the client disconnects; system.processes exposes progress and system.query_log records completion details.

2. 2

   Parquet dictionary-filter pushdown skips row groups whose dictionaries cannot match a filter; the setting defaults to a 1 MiB dictionary-page limit and can be disabled with value 0.

3. 3

   Small-table column statistics are now built on INSERT by default, producing a reported 29% improvement across all TPC-H benchmarks.