ClickHouse release 26.6

Backward Incompatible Changes

• Removed allow_experimental_query_deduplication feature after long experimental period with no tests or support.
• XRay-based SYSTEM INSTRUMENT feature renamed from parameters to arguments: system.instrumentation.parameters → arguments, and SYSTEM INSTRUMENT ADD ... HANDLER PARAMETERS ... → SYSTEM INSTRUMENT ADD ... HANDLER ARGUMENTS ...
• Setting show_data_lake_catalogs_in_system_tables renamed to show_remote_databases_in_system_tables with expanded scope: when set to 0 (default), MySQL and PostgreSQL databases are also hidden from system tables (old name kept as alias).
• Default x86 build now targets x86-64-v3 (AVX2) instead of x86-64-v2 (SSE4.2); requires CPU with AVX2 support. Use amd64compat build for older CPUs.
• Nested Dynamic/Variant now rejected in min/max aggregates and minmax indexes.
• icebergHash and icebergBucket now explicitly reject Int128, UInt128, Int256, UInt256, and Decimal256 arguments.
• --echo CLI option now accepts optional boolean value; positional queries after bare --echo treated as its value; use --echo --query "..." or --echo=false instead.
• Removed experimental KQL functions array_sort_asc, array_sort_desc, and their SQL backends.
• ALTER TABLE ... REPLACE PARTITION ... FROM ... now rejects requests when source table has no parts in requested partition (previously silently dropped destination data). Set allow_replace_partition_from_empty_source = 1 to restore old behavior.
• Default insert_deduplication_version changed from compatible_double_hashes to new_unified_hash; deduplication now per-insert instead of per-part/partition.

New Features

• Continuous queries over MergeTree tables via snapshot reads (first step toward streaming query support).
• Hypothetical (what-if) skip indexes with CREATE HYPOTHETICAL INDEX and EXPLAIN WHATIF for cost estimation.
• Embedded /schema web UI visualizing dependency graph between tables, views, materialized views, dictionaries, and distributed tables.
• PNG output format support for rendering query results as images.
• AI coding agent detection (Claude Code, Cursor, Codex, etc.) reported in system.query_log, system.query_thread_log, and system.processes.
• New system.documentation table with embedded reference documentation for functions, table engines, data types, settings, and formats.
• GeoJSON input format for reading FeatureCollection documents with native Geometry type support.
• Mapbox Vector Tile functions: MVTEncodeGeom, MVTEncode, MVTBoundingBox, MVTBoundingBoxMercator (with PostGIS aliases).
• clickhouse-local now supports SYSTEM START LISTEN and SYSTEM STOP LISTEN queries.
• Interactive help command in clickhouse-client and clickhouse-local with syntax-highlighted documentation.
• Azure Data Lake Storage Gen2 write support.
• RowBinaryWithNamesAndTypesAndDefaults format for better schema evolution support.
• ADD ENUM VALUES in ALTER TABLE to append new enum values without respecifying all current values.
• Refreshable materialized view REFRESH DEPENDS ON for dependency-triggered refreshes.
• Column selection by name pattern: * LIKE '<pattern>' and * ILIKE '<pattern>'.
• ESCAPE syntax for custom escape characters in LIKE patterns.
• materialize_projections_on_insert and materialize_projections_on_merge MergeTree settings.
• allow_tuple_element_aggregation setting for recursive tuple flattening in aggregating engines.
• quantizeBFloat16ToInt8 and dequantizeInt8ToBFloat16 scalar codec functions.
• arrayTopK and arrayBottomK array functions.
• system.iceberg_files table exposing per-file metadata for Iceberg tables.
• system.constraints table for CHECK and ASSUME constraints.
• system.dictionary_layouts, system.dictionary_sources, system.data_skipping_index_types, system.disk_types tables with embedded documentation.
• SYSTEM RESTART DISK command for reloading disk metadata and re-scanning data parts.
• PostgreSQL-style expr OP SOME(array) / expr OP ALL(array) syntax.
• hostname_longest_common_prefix and hostname_longest_common_suffix load balancing strategies.
• TLS client certificate information in system.session_log.
• Optional external_id credential for S3 role-based access.
• S3Queue setting after_processing_move_preserve_path for preserving source paths during moves.
• message_queue_disable_insertion setting to disable insertion from message queue engines.
• LOCALTIME and LOCALTIMESTAMP SQL-standard functions.
• date_part() function as syntactic sugar for EXTRACT().
• PostgreSQL-compatible EXTRACT(TIMEZONE_HOUR/MINUTE FROM dt) and interval extraction.
• min_by and max_by aggregate aliases for argMin and argMax.
• REGEXP_SUBSTR case-insensitive alias of regexpExtract.
• SESSION_USER case-insensitive alias of currentUser().
• Trailing NULL / NOT NULL modifier in ALTER TABLE ... ADD/MODIFY COLUMN.
• h3PolygonToCellsWithContainment for H3 polygon-to-cells conversion with containment modes.
• IPV4_PREFIX_BITS and IPV6_PREFIX_BITS flags for IP-based keying.
• formatReadableTimeDelta now accepts INTERVAL expressions other than Month/Year.
• Optional precision argument for formatReadableSize, formatReadableDecimalSize, formatReadableQuantity.
• output_format_float_precision setting for controlling decimal digits in float output.
• output_format_always_write_decimal_point_in_float_and_decimal setting.
• New HTTP handler configuration elements: <url_prefix>, <full_url_prefix>, <url_regexp>, <full_url_regexp>, <headers_regexp>.
• basic statistics (compact per-column min/max, avg string length, NULL counts).
• TTL nodes in ClickHouse Keeper (opt-in via create_ttl feature flag).
• Memory reservation feature for workloads.

Experimental Features

• Multi-stage distributed query execution with scatter/broadcast/gather/shuffle exchanges via TCP or shared object storage.
• dphyp join reordering algorithm for inner joins with query_plan_optimize_join_order_max_searched_plans setting.
• aiEmbed function for LLM-based embedding generation.
• Linux OOM canary: sacrificial child process to shed memory pressure before main server.
• Web terminal interface (/webterminal) promoted to production feature (enabled by default).
• PromQL HTTP endpoints with database/table URL query parameters.
• PromQL histogram_quantile function support in prometheusQuery and prometheusQueryRange.
• Lazy posting list apply mode for text indexes with cursor-based decoding.
• AssemblyScript ABI support for WebAssembly UDFs.
• Prometheus handlers on main HTTP port with optional prefix.
• packed_skip_index_max_bytes MergeTree setting for bundling skip-index substreams.
• Buffers serialization for WebAssembly UDFs using ABI BUFFERED_V1.
• use_reader_executor setting routing reads through new pipeline-based ReaderExecutor.
• Descending sort order in MergeTree sorting keys (no longer requires allow_experimental_reverse_key).
• Nullable(Tuple(...)) type (Beta, disabled by default).

Performance Improvements

• Keeper ~2x faster overall (better batching, message pipelining, log append pipelining).
• Simple SELECT queries ~50% faster (reduced per-query overhead for parsing, analysis, planning).
• Primary key index analysis optimized for long and high-cardinality keys.
• New ipnsort and driftsort stable sorting implementations for faster ORDER BY.
• GROUP BY optimization for high cardinality evenly distributed keys via row scattering (enable_sharding_aggregator).
• Hash table prefetching for string key GROUP BY queries (~8% improvement).
• Reduced peak memory for merging partial two-level aggregation results.
• Enum DataType memory optimization (up to 10x footprint reduction).
• Identifier resolution caching to prevent duplicate resolution.
• Query analysis ~50x faster for nested SELECT * over tables with ~1200 columns.
• Query analysis faster for queries with many/deeply nested function calls.
• Replicated access storage login/startup stalls fixed with per-entity cache batching.
• Hash join runtime filter from FixedHashMap sharing via join_runtime_filter_from_fixed_hash_table.
• Lazy selector/replication index application for JOIN + LIMIT scenarios.
• DP JOIN reordering now allowed with parallel replicas.
• JOIN runtime filter aware join-reorder cost model.
• RIGHT/FULL join non-joined row emission de-virtualized via batching.
• Packed keys32/keys64 methods in HashJoin and Set.
• Optimal nullable column handling in aggregation.
• LIMIT BY query performance improvements via multiple optimizations.
• Sharded aggregation, grace_hash join, parallel window partitioning ~2x faster (hardware CRC32C).
• Array distance functions auto-vectorized on x86-64-v4/v3.
• {Norm}Distance and dotProduct functions improved.
• arrayNorm function performance improved.
• AVX-512 const-left path improvements for distance functions.
• encrypt, decrypt, tryDecrypt, aes_encrypt_mysql, aes_decrypt_mysql up to 10x faster.
• encrypt, decrypt, halfMD5 faster via OpenSSL 3.x provider lookup avoidance.
• SHA1 SIMD implementation using AVX-512 for parallel hashing.
• AArch64 ASIMD/NEON backend for multi-buffer MD5.
• MATERIALIZE PROJECTION ~3.4x faster via pre-squashing.
• MergeTree INSERT latency reduced via skip index column permutation optimization.
• Parquet BYTE_STREAM_SPLIT decoding faster via Arrow SIMD paths.
• LowCardinality insertion performance improved for bloom_filter indexes.
• Blob copy parallelization in DiskObjectStorageTransaction.
• One input format avoids file content reading.
• macOS startup ~3x faster (no dynamic symbol export).
• Filesystem cache loading faster via redundant directory open elimination.
• Filesystem cache loading faster via idle thread re-utilization.
• Startup overhead reduced via WasmEdge library update.
• MergeTree primary key and skip indexes prune granules for ifNull/coalesce wrapped conditions.
• DateTime64 skip index evaluation consumes less CPU.
• Text index multi-token search analysis optimized for rare tokens.
• Text index posting-list segment caching.
• Generic exclusion search performance improved.
• S3 client endpoint/bucket region discovery caching.
• has() with constant arrays transformed to faster in() implementation.
• ThreadPool LIFO wake strategy reduces memory fragmentation.
• Approximate runtime filter and bloom filter index performance improved.
• ASOF JOIN can now use parallel_hash algorithm.
• Early termination for empty INTERSECT/EXCEPT results.
• bitmapContains for non-UInt64 groupBitmap optimized.
• Faster discontinuous queries for single-dictionary LowCardinality columns.
• HashJoin probe fast path for single-row blocks.
• Predicate pushdown for remote table function queries.
• enable_join_transitive_predicates enabled by default.
• Text index support for multiSearchAny, multiSearchAnyUTF8, multiMatchAny, improved match pattern analysis.
• Filter merging into PREWHERE on second optimizer pass.
• QueryConditionCache records individually filtered-out granules.
• Keeper nuraft_use_bg_thread_for_snapshot_io enabled by default.
• Keeper snapshot application peak memory usage reduced.
• LLVM/JIT allocations routed to dedicated jemalloc arena.
• Excessive memory reservation for sparse dictionary-encoded Nullable(String) Parquet columns fixed.
• Cardinality estimation improved for deterministic single-argument functions in join reordering.
• Asynchronous logging CPU overhead reduced via LIFO queue notifications.
• Duplicate calculation removal during query execution.
• Recursive CTE result processing parallelized.
• Case-insensitive substring search faster via NEON (ARM) and wider AVX2 (x86).
• FPC floating-point codec ARM regression fixed.

Improvements

• Dynamic server hard memory limit adjustment based on current memory usage and system MemAvailable.
• C# PostgreSQL client support in PostgreSQL protocol.
• Mutations now use the analyzer.
• Schema evolution with concurrent consistency checks for data lake catalogs.
• Cache disk support for datalake table engines.
• DataLakeCatalog onelake type now uses Blob endpoint by default.
• S3(Azure)Queue default persistent_processing_node_ttl_seconds increased from 1 hour to 6 hours.
• Iceberg metadata cache default size reduced from 1 GB to 128 MB.
• S3Queue streaming task query_id propagated to dependent table inserts.
• system.query_log used_storages populated with storage engine name for DataLakeCatalog queries.
• Iceberg snapshot operation and summary information now accessible.
• S3 multipart upload finalization completed asynchronously via task tracker.
• MergeTree text index default parameter settings: text_index_dictionary_block_size, text_index_dictionary_block_frontcoding_compression, text_index_posting_list_block_size, text_index_posting_list_codec.
• Projections can override additional MergeTree settings (compression, part format, serialization).
• system.merges new columns: current_projection, current_projection_progress, projections_completed, projections_remaining.
• PREWHERE with IN subquery on primary key columns now uses primary key index for granule pruning.
• h3PolygonToCells enforces max array size, validates H3 return codes, rejects MultiLineString.
• system.keeper_snapshots table with local Keeper snapshot information.
• system.keeper_changelogs table with Keeper changelog (Raft log) file information.
• system.keeper_cluster table with Raft cluster member information.
• Keeper profile events expanded for watches.
• Keeper coordination_settings for NuRaft uncommitted log entry admission limiting and append-entries throttling.
• enable_compression setting for mysql table function, MySQL table/database engines, and MySQL dictionary source.
• Query cancellation latency reduced for PostgreSQL wire protocol.
• Query cancellation latency reduced for MySQL wire protocol.
• MySQL handshake auth_response length reading fixed for values >= 128.
• MaterializedPostgreSQL maps PostgreSQL numeric(p, 0) with precision > 76 to Int256.