system.part_log

System table containing information about events that occurred with data parts in the MergeTree family tables, such as adding or merging of data.

Querying in ClickHouse Cloud

The data in this system table is held locally on each node in ClickHouse Cloud. Obtaining a complete view of all data, therefore, requires the clusterAllReplicas function. See here for further details.

Description

The system.part_log table is created only if the part_log server setting is specified.

This table contains information about events that occurred with data parts in the MergeTree family tables, such as adding or merging data.

The system.part_log table contains the following columns:

Columns

• hostname (LowCardinality(String)) — Hostname of the server executing the query.
• query_id (String) — Identifier of the INSERT query that created this data part.
• event_type (Enum8) — Type of the event that occurred with the data part. Can have one of the following values:
  • NewPart — Inserting of a new data part.
  • MergePartsStart — Merging of data parts has started.
  • MergeParts — Merging of data parts has finished.
  • DownloadPart — Downloading a data part.
  • RemovePart — Removing or detaching a data part using DETACH PARTITION.
  • MutatePartStart — Mutating of a data part has started.
  • MutatePart — Mutating of a data part has finished.
  • MovePart — Moving the data part from the one disk to another one.
• merge_reason (Enum8) — The reason for the event with type MERGE_PARTS. Can have one of the following values:
  • NotAMerge — The current event has the type other than MERGE_PARTS.
  • RegularMerge — Some regular merge.
  • TTLDeleteMerge — Cleaning up expired data.
  • TTLRecompressMerge — Recompressing data part with the.
• merge_algorithm (Enum8) — Merge algorithm for the event with type MERGE_PARTS. Can have one of the following values:
  • Undecided
  • Horizontal
  • Vertical
• event_date (Date) — Event date.
• event_time (DateTime) — Event time.
• event_time_microseconds (DateTime64) — Event time with microseconds precision.
• duration_ms (UInt64) — Duration.
• database (String) — Name of the database the data part is in.
• table (String) — Name of the table the data part is in.
• table_uuid (UUID) — UUID of the table the data part belongs to.
• part_name (String) — Name of the data part.
• partition_id (String) — ID of the partition that the data part was inserted to. The column takes the all value if the partitioning is by tuple().
• partition (String) - The partition name.
• part_type (String) - The type of the part. Possible values: Wide and Compact.
• disk_name (String) - The disk name data part lies on.
• path_on_disk (String) — Absolute path to the folder with data part files.
• rows (UInt64) — The number of rows in the data part.
• size_in_bytes (UInt64) — Size of the data part in bytes.
• merged_from (Array(String)) — An array of names of the parts which the current part was made up from (after the merge or mutations).
• bytes_uncompressed (UInt64) — Size of uncompressed bytes.
• read_rows (UInt64) — The number of rows was read during the merge.
• read_bytes (UInt64) — The number of bytes was read during the merge.
• peak_memory_usage (Int64) — The maximum difference between the amount of allocated and freed memory in the context of this thread.
• error (UInt16) — The code number of the occurred error.
• exception (String) — Text message of the occurred error.
• mutation_ids (Array(String)) — An array of mutation IDs applied to the source part (merged_from) for the event with type MutatePartsStart and MutateParts.
• ProfileEvents (Map(String, UInt64)) — ProfileEvents that measure different metrics. The description of them can be found in the table system.events.

The system.part_log table is created after the first inserting data to the MergeTree table.

Example

SELECT * FROM system.part_log LIMIT 1 FORMAT Vertical;

Row 1:
──────
hostname:                clickhouse.eu-central1.internal
query_id:
event_type:              MergeParts
merge_reason:            RegularMerge
merge_algorithm:         Vertical
event_date:              2025-07-19
event_time:              2025-07-19 23:54:19
event_time_microseconds: 2025-07-19 23:54:19.710761
duration_ms:             2158
database:                default
table:                   github_events
table_uuid:              1ad33424-f5f5-402b-ac03-ec82282634ab
part_name:               all_1_7_1
partition_id:            all
partition:               tuple()
part_type:               Wide
disk_name:               default
path_on_disk:            ./data/store/1ad/1ad33424-f5f5-402b-ac03-ec82282634ab/all_1_7_1/
rows:                    3285726 -- 3.29 million
size_in_bytes:           438968542 -- 438.97 million
merged_from:             ['all_1_1_0','all_2_2_0','all_3_3_0','all_4_4_0','all_5_5_0','all_6_6_0','all_7_7_0']
bytes_uncompressed:      1373137767 -- 1.37 billion
read_rows:               3285726 -- 3.29 million
read_bytes:              1429206946 -- 1.43 billion
peak_memory_usage:       303611887 -- 303.61 million
error:                   0
exception:
mutation_ids:
ProfileEvents:           {'FileOpen':703,'ReadBufferFromFileDescriptorRead':3824,'ReadBufferFromFileDescriptorReadBytes':439601681,'WriteBufferFromFileDescriptorWrite':592,'WriteBufferFromFileDescriptorWriteBytes':438988500,'ReadCompressedBytes':439601681,'CompressedReadBufferBlocks':6314,'CompressedReadBufferBytes':1539835748,'OpenedFileCacheHits':50,'OpenedFileCacheMisses':484,'OpenedFileCacheMicroseconds':222,'IOBufferAllocs':1914,'IOBufferAllocBytes':319810140,'ArenaAllocChunks':8,'ArenaAllocBytes':131072,'MarkCacheMisses':7,'CreatedReadBufferOrdinary':534,'DiskReadElapsedMicroseconds':139058,'DiskWriteElapsedMicroseconds':51639,'AnalyzePatchRangesMicroseconds':28,'ExternalProcessingFilesTotal':1,'RowsReadByMainReader':170857759,'WaitMarksLoadMicroseconds':988,'LoadedMarksFiles':7,'LoadedMarksCount':14,'LoadedMarksMemoryBytes':728,'Merge':2,'MergeSourceParts':14,'MergedRows':3285733,'MergedColumns':4,'GatheredColumns':51,'MergedUncompressedBytes':1429207058,'MergeTotalMilliseconds':2158,'MergeExecuteMilliseconds':2155,'MergeHorizontalStageTotalMilliseconds':145,'MergeHorizontalStageExecuteMilliseconds':145,'MergeVerticalStageTotalMilliseconds':2008,'MergeVerticalStageExecuteMilliseconds':2006,'MergeProjectionStageTotalMilliseconds':5,'MergeProjectionStageExecuteMilliseconds':4,'MergingSortedMilliseconds':7,'GatheringColumnMilliseconds':56,'ContextLock':2091,'PartsLockHoldMicroseconds':77,'PartsLockWaitMicroseconds':1,'RealTimeMicroseconds':2157475,'CannotWriteToWriteBufferDiscard':36,'LogTrace':6,'LogDebug':59,'LoggerElapsedNanoseconds':514040,'ConcurrencyControlSlotsGranted':53,'ConcurrencyControlSlotsAcquired':53}