system.part_log

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.

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:

• 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).
• 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.
• 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:
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}