ClickHouse release 26.5

Backward Incompatible Changes

• Changed defaults of date_time_input_format and cast_string_to_date_time_mode from basic to best_effort. Set to basic or compatibility to restore old parsing behavior.
• Tuple element name null is now forbidden due to conflicts with Nullable subcolumn names.
• Added settings dynamic_disk_allow_from_env, dynamic_disk_allow_from_zk, dynamic_disk_allow_include that disallow these operations by default in dynamic disks.
• Removed obsolete Arrow-based Parquet reader/writer; native implementation is used instead.
• SHOW CREATE TABLE t now prefers temporary tables when both exist; DESCRIBE TEMPORARY TABLE syntax is now supported.
• Reduced default http_max_fields from 1,000,000 to 1,000 and http_max_field_name_size from 128 KB to 4 KB to limit pre-authentication memory usage.
• Added histograms nested column to system.metric_log; deprecates system.histogram_metric_log table.
• CAST to DateTime or DateTime64 without explicit timezone now preserves source timezone.
• Removed kql table function; use SET dialect = 'kusto' instead.
• Window functions RANK and DENSE_RANK now reject arguments per SQL standard.

New Features

• Added max_bytes_ratio_before_external_join setting (default 0.5) for hash join spill-to-disk threshold as fraction of available memory.
• Added filesystem table function to represent directory structure as queryable table.
• Allow passing bare function names to higher-order functions like arrayMap, arrayFilter.
• Added send_table_structure_on_insert_with_inline_data setting and --inline-insert-data client option.
• Added tokenizeQuery and highlightQuery functions for SQL query tokenization and syntax highlighting.
• Added url_base setting to resolve relative URLs in url table function and URL table engine.
• Support negative values in LIMIT BY clause to select rows from end of each group.
• Support tuple and array output in json_value function.
• Added kafka_autodetect_client_rack parameter to discover Availability Zone and propagate as client rack.
• Added Read event type to system.blob_storage_log for tracking object storage reads.
• Users can now see ZooKeeper watches via new system.zookeeper_watches table.
• Added Shards profile event counting shards in distributed queries.
• WASM UDFs can be declared DETERMINISTIC for constant folding.
• Added parallel_replicas_prefer_local_replica setting to control replica selection.
• Added socket buffer size settings: {disk,storage,http}_connections_{rcvbuf,sndbuf}.
• Support CREATE OR REPLACE MATERIALIZED VIEW with atomic swap semantics.
• Added S3 histogram metrics s3_read_request_duration_microseconds and s3_read_request_bytes.
• Added Paimon, PaimonS3, PaimonAzure, PaimonHDFS, PaimonLocal table engines with incremental read support.
• Added Kafka kafka_map_virtual_columns_on_write setting to map virtual columns on insert.
• Added SYSTEM PAUSE VIEW and SYSTEM PAUSE VIEWS for refreshable materialized views.
• Added regexpPosition function (PostgreSQL-compatible aliases regexpInstr, regexp_instr) for regex match position.
• New functions isPrime and isProbablePrime for primality checks.
• clear and /clear now clear the terminal in CLI tools.
• Allow file table function to accept Array(String) of paths.
• Added deterministic and higher_order columns to system.functions.
• Added optional encoding variant parameter to bech32Encode and raw decode mode to bech32Decode.
• Can now write data in AvroConfluent format with output_format_avro_confluent_subject setting.
• Added prettyPrintJSON function to format JSON strings into human-readable form.
• Added STRING_AGG as case-insensitive alias of groupConcat.
• Individual subquery results can be cached independently with use_query_cache per subquery.
• Added lexer-based syntax highlighting to web UI query editor.

Experimental Features

• Added experimental web terminal interface at /webterminal with interactive clickhouse-client over WebSocket.
• Kafka2 engine now supports direct SELECT queries and kafka_commit_on_select setting.
• WASM UDFs support more numeric type coercions.
• Support LIKE function in DELETE FROM system.webassembly_modules.
• Support geo types for Iceberg.
• Add prepared statements functionality to ArrowFlight SQL server.
• Improved KQL parser robustness.

Performance Improvements

• Reuse Parquet footer metadata cache for local files.
• Push ORDER BY ... LIMIT n through LEFT/RIGHT joins when applicable.
• Enable use_top_k_dynamic_filtering and use_skip_indexes_for_top_k by default.
• Use jemalloc oversize-arena feature to reduce page faults.
• Limit simultaneously active streams in UNION ALL to reduce peak memory.
• Optimize userspace page cache; cold reads through object storage page cache now coalesce consecutive misses into single HTTP request.
• Speed up index analysis with type casting and function application.
• Speed up queries on Merge tables with many underlying tables.
• Remove vtable from settings, reducing Settings copy size by ~28x.
• Added max_threads_min_free_memory_per_thread and max_insert_threads_min_free_memory_per_thread settings for automatic parallelism reduction.
• Reduce memory usage on low-memory systems (<4 GiB).
• Added OptimizeTrivialGroupByLimitPass for trivial GROUP BY queries.
• Granule-level implicit min-max index for _part_offset and _block_number virtual columns.
• Precompute per-row hashes for prealloc_serialized aggregation methods.
• Query condition cache for Iceberg tables.
• Optimize cramersV and related functions with window functions.
• Rewrite tupleElement(dictGet(...), N) optimization.
• Enable buffering for sorting steps in distributed queries.
• Performance improvements for partial_merge JOIN.
• Reduced memory over-allocation in partial_merge JOIN.
• Optimize memory tracker sampling.
• Reduce memory allocation during S3 multipart uploads.
• Add software prefetch in hash join probe phase.
• Optimize MemoryTracker layout improving performance by ~25%.
• Rewrite coalesce(a, b, ...) <op> const predicates for better index pruning.
• Significantly improved Iceberg catalog query performance with large JSON metadata.
• Avoid index uncompressed cache overhead when disabled.
• Fix excessive granule reading in ORDER BY ... LIMIT with non-selective filters.
• Fixed missing hash table sizes cache reuse after JOIN reordering.
• Fsync of part files now performed in parallel, speeding up insert finalization by ~22%.
• Vectorize string search helpers on AArch64 using NEON.
• Restore I/O savings for aliased complex columns.
• Reduced lock contention on asynchronous remote-FS read path.
• Slightly better query plan for projections in-order scan.
• Extend projection optimization to handle cases where projection's sorting key removes sorting step.
• Allow json.path[] syntax sugar on typed JSON paths.
• Optimize json.path[N].nested.path expansion.
• Optimize SHOW TABLES access checks.
• Remove redundant per-table access check in system.tables fast path.
• Enable JIT compilation on macOS.
• Skip row policy deferral after FINAL when deterministic and depends only on sorting-key columns.
• Support hasAny and hasAll as text index filter predicates.
• Route long-lived MergeTree heap state to dedicated jemalloc arena.
• Improve ZooKeeper client timeout handling under heavy load with progress-based timeout.
• Move per-event trace flags into separate array with lazy allocation.
• Add compareTrackAt for ColumnDecimal.
• Added SIMD implementation of MD5 (AVX2/AVX512) for parallel hashing.

Improvements

• Improved --help output for all ClickHouse applications with better formatting and subcommand listing.
• Changed default input_format_column_name_matching_mode from match_case to auto for case-insensitive fallback.
• Added STDDEV alias of stddevSamp.
• Added array_to_string alias of arrayStringConcat.
• Added unnest alias of arrayJoin.
• Progress bar in clickhouse-client now shows temporary disk data usage.
• Added system.predicate_statistics_log for predicate filter selectivity sampling.
• Allow skipping local shard with missing table when skip_unavailable_shards enabled.
• Added startup warnings for Linux mdraid array issues.
• WASM UDFs now appear in system.functions with correct metadata.
• generate_series table function supports negative step values.
• Fixed context expiration in functions used in deferred execution paths.
• Fixed spurious cache limit violations.
• Auto-detect region for S3 Express endpoints.
• Support field IDs for Iceberg data files.
• Left panel in web UI scrolls independently and stays visible.
• Allow opening Play UI in new tab via Ctrl/Cmd/Shift+click.
• New metrics for projection primary key and index granularity memory usage.
• Apply network bandwidth limits to remote filesystem operations.
• Improved backup resilience for Refreshable Materialized Views.
• Consistent database listing in system.databases regardless of catalog settings.
• Aligned concurrent_threads_soft_limit_ratio_to_cores default in code with config.
• Improved sanitizer robustness in parser and string functions.
• Improved monotonicity inference for divide(0, x) and intDiv(0, x).
• Replaced HTTPConnectionPool async metrics with bucket-based histogram.
• Socket buffer settings now reported as changeable without restart.
• Added symbol resolution support in flameGraph.
• CLI client supports <rainbow_parentheses>false</rainbow_parentheses> config.
• Fixed MultiVolnitsky UTF-8 case-insensitive search.
• Implemented sched_getcpu via rseq TLS in glibc-compatibility musl.
• Replicated refreshable materialized views no longer refresh twice on ZK reconnection.
• Allow Distributed tables without explicit column list to validate sharding keys.
• Fixed REST catalog with Azure abfss paths.
• Honor role_arn and role_session_name in Keeper S3 snapshot client.
• Fixed memory leak in MATERIALIZED CTEs.
• Fixed exception in lowerUTF8/upperUTF8 with large non-ASCII datasets.
• Hardened MergeTree column-statistics loading against transient I/O failures.
• INSERT INTO ... SELECT FROM input(...) no longer requires CREATE TEMPORARY TABLE grant.
• Added concurrent_part_removal_threshold_for_remote_disk setting (default 16).
• New setting defer_partition_pruning_after_final for opt-out of 26.3 behavior.
• Fixed intExp2 returning wrong results for out-of-range inputs.
• Fixed shell argument quoting in clickhouse install and clickhouse git-import.
• Alias table engine is now non-experimental.
• Added --queries-format option to clickhouse-benchmark.
• Tuple element-wise operators are now variadic.
• Added /processors-profile web UI for pipeline visualization.
• Setting query_plan_use_logical_join_step is now obsolete.
• Pre-filled user input removed from web UI pages.
• Decoupled ORC reader offset-based read from use_prefetch.
• Downloaded CSV files now use .csv extension.
• More input validation for BigLake catalog.

Bug Fixes

• Fixed inconsistent expression formatting with parenthesis tracking.
• Fixed accurate comparison between Decimal and Float types.
• Allow positional arguments in distributed queries.
• Fixed assertion failure when dropping/recreating materialized views and view dependencies.
• Fixed server abort when creating table with duplicate UUID.
• Fixed skip index usage on data read with patch parts from lightweight updates.
• Fixed "Not-ready Set" exception with IN subqueries moved to PREWHERE.
• Fixed "Cannot find column" error combining ADD and RENAME COLUMN.
• Fixed views with mixed set operations returning wrong results after detach/attach.
• Fixed PLACEHOLDER action exception in stress testing.
• Fixed logical error with Iceberg Date partitions.
• Fixed skip index incompatibility after column type modification.
• Fixed credential leak in query logs.
• Fixed DROP TABLE on Kafka tables potentially hanging indefinitely.
• Fixed runtime lookup for source table engines and database source grants.
• Fixed server crash with ALIAS column containing correlated subqueries.
• Fixed server crash with correlated subqueries and CONSTRAINT ASSUME.
• Fixed exception with JSON dynamic fields as GROUP BY key.
• Fixed workload IO scheduling bypass for S3/object-storage writes.
• Fixed Protobuf/ProtobufList schema cache collision.
• Fixed heap-buffer-overflow in Rust CXX bridge.
• Fixed FunctionVariantAdaptor exception handling.
• Fixed TRUNCATE ALL TABLES with views.
• Fixed optimize_rewrite_array_exists_to_has type compatibility and re-enabled.
• Fixed S3 settings priority for storage configuration.
• Fixed sparse serialization losing negative zero sign.
• Fixed WKT geometry parser MULTILINESTRING corruption and malformed string handling.
• Fixed GROUPING SETS on distributed single-shard tables.
• Fixed LOGICAL_ERROR column identifier already registered.
• Fixed LOGICAL_ERROR with additional_result_filter on UNION/EXCEPT.
• Fixed server crash with * APPLY on aggregate functions with grouping.
• Fixed infinite loop with CSV/TSV skip first lines exceeding file lines.