docs/en/operations/system-tables/part_log.md
import SystemTableCloud from '@site/docs/_snippets/_system_table_cloud.md';
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:
UndecidedHorizontalVerticalevent_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}