Issues

• Join reordering without row estimates builds the 600M-row lineitem: TPC-H Q5 on Iceberg with grace_hash is 3.5-4x slower since DPsub became the default

  comp-joinscomp-query-optimizerperformance
  ClickHouse/ClickHouse#129290 · Filter by author alexbakharew opened on Nov 4, 2026 · Updated on Nov 4, 2026
  1

  

• Window functions batch execution (#127389): every batch walks all retained blocks

  comp-window-functions
  ClickHouse/ClickHouse#127659 · by Filter by author groeneai was closed on Nov 4, 2026 · Updated on Nov 4, 2026
  2

• AsynchronousMetrics logs "Cannot read from file .../cpu.max, errno: 19" every second after systemd disables the cgroup cpu controller

  bugcomp-loggingexternal
  ClickHouse/ClickHouse#128137 · by Filter by author bonnyfacy was closed on Nov 4, 2026 · Updated on Nov 4, 2026
  3

• Vectorise MD5 with multi-buffer SIMD implementation

  comp-regular-functionexternalperformance
  ClickHouse/ClickHouse#104887 · by Filter by author khvn26 was closed on Nov 4, 2026 · Updated on Nov 4, 2026
  6

• Parquet: a row group is decoded one block at a time, so queries that need only a few row groups leave threads idle; decoding its blocks in parallel lanes makes ClickBench Q38 (0-based) 21% faster

  comp-parquet-reader-v3
  ClickHouse/ClickHouse#129174 · Filter by author snelheidai opened on Nov 4, 2026 · Updated on Nov 4, 2026
  5

• EXPLAIN PLAN tree = 1: draw the nested plans of a step

  comp-query-optimizerunfinished code
  ClickHouse/ClickHouse#129278 · Filter by author diegomestre2 opened on Nov 4, 2026 · Updated on Nov 4, 2026
  

• Plan-based parallel replicas: IN set backed by a temporary table is shipped to the replicas

  comp-parallel-replicas
  ClickHouse/ClickHouse#129273 · Filter by author devcrafter opened on Nov 4, 2026 · Updated on Nov 4, 2026

• Settings history records of defaults changed in 26.8 are dated 26.9/26.10 on master and release/26.10, so compatibility='26.8' reverts them after the upgrade

  backward compatibilityblockerbugcomp-settingsv26.10-affected
  ClickHouse/ClickHouse#129014 · Filter by author rahulmalik87 opened on Nov 3, 2026 · Updated on Nov 4, 2026
  1

• a number first seen past row 25 000 is read as a date

  ai_p2comp-datetimeexternalpotential bug
  ClickHouse/ClickHouse#123783 · by Filter by author hanke580 was closed on Nov 4, 2026 · Updated on Nov 4, 2026
  2

• SHOW CREATE DATABASE and SELECT * FROM system.databases fail with SYNTAX_ERROR for a Filesystem, S3 or HDFS database whose path or URL has a quote

  comp-database-enginesexternal
  ClickHouse/ClickHouse#127924 · by Filter by author buggins was closed on Nov 4, 2026 · Updated on Nov 4, 2026

• Improve usability of clickhouse-keeper-client

  comp-keeperusability
  ClickHouse/ClickHouse#65034 · Filter by author alexey-milovidov opened on Jun 10, 2024 · Updated on Nov 4, 2026
  1

• An empty table JOIN with a large table consumes an unbounded amount of memory while it can be optimized

  comp-joinsexternalperformance
  ClickHouse/ClickHouse#80480 · Filter by author jinhui-lai opened on May 19, 2025 · Updated on Nov 4, 2026
  5

• Investigate unstable execution time of some queries with parallel replicas

  comp-parallel-replicasperformance
  ClickHouse/ClickHouse#89479 · Filter by author nickitat opened on Nov 4, 2025 · Updated on Nov 4, 2026
  1

• Join reorder puts the fact table on the hash build side when a dimension table has no statistics (parts written before 26.8): up to 30x slower than 26.6

  comp-query-optimizerperformance
  ClickHouse/ClickHouse#129254 · Filter by author fm4v opened on Nov 4, 2026 · Updated on Nov 4, 2026
  4

• TRUNCATE removes parts immediately, ignoring old_parts_lifetime

  comp-mergetree
  ClickHouse/ClickHouse#129256 · Filter by author alexey-milovidov opened on Nov 4, 2026 · Updated on Nov 4, 2026
  5

  

• Rewrite rdkafka

  comp-message-queuesdevelopment
  ClickHouse/ClickHouse#127470 · Filter by author alexey-milovidov opened on Oct 22, 2026 · Updated on Nov 4, 2026
  

• Performance comparison: audit of the recent growth of the master_head jobs (long queries, duplicates, amd vs arm)

  comp-ci-infrastructure
  ClickHouse/ClickHouse#129258 · Filter by author groeneai opened on Nov 4, 2026 · Updated on Nov 4, 2026
  2

• Iceberg: use the NDV of table statistics (apache-datasketches-theta-v1) in join and aggregation estimates

  comp-datalakecomp-query-optimizerfeature
  ClickHouse/ClickHouse#129260 · Filter by author antoniofilipovic opened on Nov 4, 2026 · Updated on Nov 4, 2026
  

• DEFAULT of a column added by ALTER is evaluated in the global context for rows written before the ALTER (MergeTree, Memory)

  comp-mergetree
  ClickHouse/ClickHouse#129249 · Filter by author groeneai opened on Nov 4, 2026 · Updated on Nov 4, 2026

• Unstable performance tests: 61 tests show >10% noise in >10% of master runs

  comp-testingperformance
  ClickHouse/ClickHouse#100759 · Filter by author Algunenano opened on Mar 25, 2026 · Updated on Nov 4, 2026
  28

  

• copyS3File turns a rejected zero-size CopyObject into LOGICAL_ERROR

  comp-object-storagepotential bug
  ClickHouse/ClickHouse#129218 · Filter by author alsugiliazova opened on Nov 4, 2026 · Updated on Nov 4, 2026
  2

• MarkCache: single global mutex serialises concurrent cache hits under high-QPS point lookups

  comp-mergetreeexternalperformance
  ClickHouse/ClickHouse#124444 · Filter by author ligiamonne opened on Oct 7, 2026 · Updated on Nov 4, 2026
  2

• Experiment: fix the hot code layout with a linker symbol ordering file

  buildcomp-ci-infrastructure
  ClickHouse/ClickHouse#124574 · by Filter by author Algunenano was closed on Nov 4, 2026 · Updated on Nov 4, 2026
  7

  

• Implement serde for FillingStep and support it for makeDistributed

  comp-query-optimizerunfinished code
  ClickHouse/ClickHouse#109309 · Filter by author alesapin opened on Jul 3, 2026 · Updated on Nov 4, 2026
  1

  

• Wide parts with a Geometry column written before 26.8 are unreadable after the upgrade: Stream g.MultiPoint.size0 ... is not found

  ai_p1comp-datatype-wrapper
  ClickHouse/ClickHouse#128592 · by Filter by author fm4v was closed on Nov 4, 2026 · Updated on Nov 4, 2026
  3