> ## Documentation Index > Fetch the complete documentation index at: https://private-7c7dfe99-mintlify-8c05c8a2.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. > System table containing information about events that occurred with data parts in the MergeTree family tables, such as adding or merging of data. # 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](/reference/system-tables/overview#system-tables-in-clickhouse-cloud) for further details.

Description

The `system.part_log` table is created only if the [part\_log](/reference/settings/server-settings/settings#part_log) server setting is specified. This table contains information about events that occurred with [data parts](/reference/engines/table-engines/mergetree-family/custom-partitioning-key) in the [MergeTree](/reference/engines/table-engines/mergetree-family/mergetree) family tables, such as adding or merging data. The `system.part_log` table contains the following columns:

Columns

* `hostname` ([LowCardinality(String)](/reference/data-types/lowcardinality)) — Hostname of the server executing the query. * `query_id` ([String](/reference/data-types/string)) — Identifier of the INSERT query that created this data part. * `event_type` ([Enum8('NewPart' = 1, 'MergeParts' = 2, 'DownloadPart' = 3, 'RemovePart' = 4, 'MutatePart' = 5, 'MovePart' = 6, 'MergePartsStart' = 7, 'MutatePartStart' = 8)](/reference/data-types/enum)) — 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](/reference/statements/alter/partition#detach-partitionpart).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('NotAMerge' = 1, 'RegularMerge' = 2, 'TTLDeleteMerge' = 3, 'TTLRecompressMerge' = 4, 'TTLDropMerge' = 5)](/reference/data-types/enum)) — 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, TTLDropMerge — Cleaning up expired data. TTLRecompressMerge — Recompressing data part with the. * `merge_algorithm` ([Enum8('Undecided' = 0, 'Vertical' = 1, 'Horizontal' = 2)](/reference/data-types/enum)) — Merge algorithm for the event with type MERGE\_PARTS. Can have one of the following values: Undecided, Horizontal, Vertical * `event_date` ([Date](/reference/data-types/date)) — Event date. * `event_time` ([DateTime](/reference/data-types/datetime)) — Event time. * `event_time_microseconds` ([DateTime64(6)](/reference/data-types/datetime64)) — Event time with microseconds precision. * `duration_ms` ([UInt64](/reference/data-types/int-uint)) — Duration of this operation. * `database` ([String](/reference/data-types/string)) — Name of the database the data part is in. * `table` ([String](/reference/data-types/string)) — Name of the table the data part is in. * `table_uuid` ([UUID](/reference/data-types/uuid)) — UUID of the table the data part belongs to. * `part_name` ([String](/reference/data-types/string)) — Name of the data part. * `partition_id` ([String](/reference/data-types/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](/reference/data-types/string)) — The partition name. * `part_type` ([String](/reference/data-types/string)) — The type of the part. Possible values: Wide and Compact. * `part_storage_type` ([String](/reference/data-types/string)) — The type of DataPartStorage. Possible values: Packed - all files are stored in a single blob, Full - a blob per file. * `disk_name` ([String](/reference/data-types/string)) — The disk name data part lies on. * `path_on_disk` ([String](/reference/data-types/string)) — Absolute path to the folder with data part files. * `rows` ([UInt64](/reference/data-types/int-uint)) — The number of rows in the data part. * `size_in_bytes` ([UInt64](/reference/data-types/int-uint)) — Size of the data part on disk in bytes. * `merged_from` ([Array(String)](/reference/data-types/array)) — An array of the source parts names which the current part was made up from. * `bytes_uncompressed` ([UInt64](/reference/data-types/int-uint)) — Uncompressed size of the resulting part in bytes. * `read_rows` ([UInt64](/reference/data-types/int-uint)) — The number of rows was read during the merge. * `read_bytes` ([UInt64](/reference/data-types/int-uint)) — The number of bytes was read during the merge. * `peak_memory_usage` ([UInt64](/reference/data-types/int-uint)) — The maximum amount of used during merge RAM * `deduplication_block_ids` ([Array(String)](/reference/data-types/array)) — An array of block IDs used for deduplication when inserting this part. * `error` ([UInt16](/reference/data-types/int-uint)) — The error code of the occurred exception. * `exception` ([String](/reference/data-types/string)) — Text message of the occurred error. * `mutation_ids` ([Array(String)](/reference/data-types/array)) — An array of mutation IDs applied to the source part (merged\_from) for the event with type MUTATE\_PART\_START and MUTATE\_PART. * `ProfileEvents` ([Map(LowCardinality(String), UInt64)](/reference/data-types/map)) — All the profile events captured during this operation. * `projections_duration_ms` ([Map(LowCardinality(String), UInt64)](/reference/data-types/map)) — Per-projection merge/rebuild duration in milliseconds. **Aliases:** * `ProfileEvents.Names` — Alias for `mapKeys(ProfileEvents)`. * `ProfileEvents.Values` — Alias for `mapValues(ProfileEvents)`. * `name` — Alias for `part_name`. The `system.part_log` table is created after the first inserting data to the `MergeTree` table.

Example

```sql theme={null} SELECT * FROM system.part_log LIMIT 1 FORMAT Vertical; ``` ```text theme={null} 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} ```