Altinity · zvonand · Mar 4, 2026 · Mar 4, 2026 · Mar 5, 2026 · Mar 7, 2026
diff --git a/ci/jobs/scripts/integration_tests_configs.py b/ci/jobs/scripts/integration_tests_configs.py
@@ -42,6 +42,7 @@ class TC:
     TC("test_storage_iceberg_no_spark/", True, "no idea why i'm sequential"),
     TC("test_storage_iceberg_with_spark_cache/", True, "no idea why i'm sequential"),
     TC("test_storage_iceberg_concurrent/", True, "no idea why i'm sequential"),
+    TC("test_export_replicated_mt_partition_to_object_storage/", True, "ZooKeeper can't handle too many parallel requests"),
 ]
 
 IMAGES_ENV = {

diff --git a/docs/en/engines/table-engines/mergetree-family/part_export.md b/docs/en/engines/table-engines/mergetree-family/part_export.md
@@ -0,0 +1,307 @@
+# ALTER TABLE EXPORT PART
+
+## Overview
+
+The `ALTER TABLE EXPORT PART` command exports individual MergeTree data parts to object storage (S3, Azure Blob Storage, etc.), typically in Parquet format. A commit file is shipped to the same destination directory containing all data files exported within that transaction.
+
+**Key Characteristics:**
+- **Experimental feature** - must be enabled via `allow_experimental_export_merge_tree_part` setting
+- **Asynchronous** - executes in the background, returns immediately
+- **Ephemeral** - no automatic retry mechanism; manual retry required on failure
+- **Idempotent** - safe to re-export the same part (skips by default if file exists)
+- **Preserves sort order** from the source table
+
+## Syntax
+
+```sql
+ALTER TABLE [database.]table_name 
+EXPORT PART 'part_name' 
+TO TABLE [destination_database.]destination_table 
+SETTINGS allow_experimental_export_merge_tree_part = 1 
+         [, setting_name = value, ...]
+```
+
+## Syntax with table function
+
+```sql
+ALTER TABLE [database.]table_name
+EXPORT PART 'part_name'
+TO TABLE FUNCTION s3(s3_conn, filename='table_function', partition_strategy...)
+SETTINGS allow_experimental_export_merge_tree_part = 1
+         [, setting_name = value, ...]
+```
+
+### Parameters
+
+- **`table_name`**: The source MergeTree table containing the part to export
+- **`part_name`**: The exact name of the data part to export (e.g., `'2020_1_1_0'`, `'all_1_1_0'`)
+- **`destination_table`**: The target table for the export (typically an S3, Azure, or other object storage table)
+
+## Requirements
+
+Source and destination tables must be 100% compatible:
+
+1. **Identical schemas** - same columns, types, and order
+2. **Matching partition keys** - partition expressions must be identical
+
+In case a table function is used as the destination, the schema can be omitted and it will be inferred from the source table.
+
+## Settings
+
+### `allow_experimental_export_merge_tree_part` (Required)
+
+- **Type**: `Bool`
+- **Default**: `false`
+- **Description**: Must be set to `true` to enable the experimental feature.
+
+### `export_merge_tree_part_overwrite_file_if_exists` (Optional)
+
+- **Type**: `Bool`
+- **Default**: `false`
+- **Description**: If set to `true`, it will overwrite the file. Otherwise, fails with exception.
+
+### `export_merge_tree_part_max_bytes_per_file` (Optional)
+
+- **Type**: `UInt64`
+- **Default**: `0`
+- **Description**: Maximum number of bytes to write to a single file when exporting a merge tree part. 0 means no limit. This is not a hard limit, and it highly depends on the output format granularity and input source chunk size. Using this might break idempotency, use it with care.
+
+### `export_merge_tree_part_max_rows_per_file` (Optional)
+
+- **Type**: `UInt64`
+- **Default**: `0`
+- **Description**: Maximum number of rows to write to a single file when exporting a merge tree part. 0 means no limit. This is not a hard limit, and it highly depends on the output format granularity and input source chunk size. Using this might break idempotency, use it with care.
+
+### export_merge_tree_part_throw_on_pending_mutations
+
+- **Type**: `bool`
+- **Default**: `true`
+- **Description**: If set to true, throws if pending mutations exists for a given part. Note that by default mutations are applied to all parts, which means that if a mutation in practice would only affetct part/partition x, all the other parts/partition will throw upon export. The exception is when the `IN PARTITION` clause was used in the mutation command. Note the `IN PARTITION` clause is not properly implemented for plain MergeTree tables.
+
+### export_merge_tree_part_throw_on_pending_patch_parts
+
+- **Type**: `bool`
+- **Default**: `true`
+- **Description**: If set to true, throws if pending patch parts exists for a given part. Note that by default mutations are applied to all parts, which means that if a mutation in practice would only affetct part/partition x, all the other parts/partition will throw upon export. The exception is when the `IN PARTITION` clause was used in the mutation command. Note the `IN PARTITION` clause is not properly implemented for plain MergeTree tables.
+
+## Examples
+
+### Basic Export to S3
+
+```sql
+-- Create source and destination tables
+CREATE TABLE mt_table (id UInt64, year UInt16) 
+ENGINE = MergeTree() PARTITION BY year ORDER BY tuple();
+
+CREATE TABLE s3_table (id UInt64, year UInt16) 
+ENGINE = S3(s3_conn, filename='data', format=Parquet, partition_strategy='hive') 
+PARTITION BY year;
+
+-- Insert and export
+INSERT INTO mt_table VALUES (1, 2020), (2, 2020), (3, 2021);
+
+ALTER TABLE mt_table EXPORT PART '2020_1_1_0' TO TABLE s3_table 
+SETTINGS allow_experimental_export_merge_tree_part = 1;
+
+ALTER TABLE mt_table EXPORT PART '2021_2_2_0' TO TABLE s3_table 
+SETTINGS allow_experimental_export_merge_tree_part = 1;
+```
+
+### Table function export
+
+```sql
+-- Create source and destination tables
+CREATE TABLE mt_table (id UInt64, year UInt16)
+ENGINE = MergeTree() PARTITION BY year ORDER BY tuple();
+
+-- Insert and export
+INSERT INTO mt_table VALUES (1, 2020), (2, 2020), (3, 2021);
+
+ALTER TABLE mt_table EXPORT PART '2020_1_1_0' TO TABLE FUNCTION s3(s3_conn, filename='table_function', format=Parquet, partition_strategy='hive') PARTITION BY year
+SETTINGS allow_experimental_export_merge_tree_part = 1;
+```
+
+## Monitoring
+
+### Active Exports
+
+Active exports can be found in the `system.exports` table. As of now, it only shows currently executing exports. It will not show pending or finished exports.
+
+```sql
+arthur :) select * from system.exports;
+
+SELECT *
+FROM system.exports
+
+Query id: 2026718c-d249-4208-891b-a271f1f93407
+
+Row 1:
+──────
+source_database:               default
+source_table:                  source_mt_table
+destination_database:          default
+destination_table:             destination_table
+create_time:                   2025-11-19 09:09:11
+part_name:                     20251016-365_1_1_0
+destination_file_paths:        ['table_root/eventDate=2025-10-16/retention=365/20251016-365_1_1_0_17B2F6CD5D3C18E787C07AE3DAF16EB1.1.parquet']
+elapsed:                       2.04845441
+rows_read:                     1138688 -- 1.14 million
+total_rows_to_read:            550961374 -- 550.96 million
+total_size_bytes_compressed:   37619147120 -- 37.62 billion
+total_size_bytes_uncompressed: 138166213721 -- 138.17 billion
+bytes_read_uncompressed:       316892925 -- 316.89 million
+memory_usage:                  596006095 -- 596.01 million
+peak_memory_usage:             601239033 -- 601.24 million
+```
+
+### Export History
+
+You can query succeeded or failed exports in `system.part_log`. For now, it only keeps track of completion events (either success or fails).
+
+```sql
+arthur :) select * from system.part_log where event_type='ExportPart' and table = 'replicated_source' order by event_time desc limit 1;
+
+SELECT *
+FROM system.part_log
+WHERE (event_type = 'ExportPart') AND (`table` = 'replicated_source')
+ORDER BY event_time DESC
+LIMIT 1
+
+Query id: ae1c1cd3-c20e-4f20-8b82-ed1f6af0237f
+
+Row 1:
+──────
+hostname:                arthur
+query_id:                
+event_type:              ExportPart
+merge_reason:            NotAMerge
+merge_algorithm:         Undecided
+event_date:              2025-11-19
+event_time:              2025-11-19 09:08:31
+event_time_microseconds: 2025-11-19 09:08:31.974701
+duration_ms:             4
+database:                default
+table:                   replicated_source
+table_uuid:              78471c67-24f4-4398-9df5-ad0a6c3daf41
+part_name:               2021_0_0_0
+partition_id:            2021
+partition:               2021
+part_type:               Compact
+disk_name:               default
+path_on_disk:            
+remote_file_paths        ['year=2021/2021_0_0_0_78C704B133D41CB0EF64DD2A9ED3B6BA.1.parquet']
+rows:                    1
+size_in_bytes:           272
+merged_from:             ['2021_0_0_0']
+bytes_uncompressed:      86
+read_rows:               1
+read_bytes:              6
+peak_memory_usage:       22
+error:                   0
+exception:               
+ProfileEvents:           {}
+```
+
+### Profile Events
+
+- `PartsExports` - Successful exports
+- `PartsExportFailures` - Failed exports
+- `PartsExportDuplicated` - Number of part exports that failed because target already exists.
+- `PartsExportTotalMilliseconds` - Total time
+
+### Split large files
+
+```sql
+alter table big_table export part '2025_0_32_3' to table replicated_big_destination SETTINGS export_merge_tree_part_max_bytes_per_file=10000000, output_format_parquet_row_group_size_bytes=5000000;
+
+arthur :) select * from system.exports;
+
+SELECT *
+FROM system.exports
+
+Query id: d78d9ce5-cfbc-4957-b7dd-bc8129811634
+
+Row 1:
+──────
+source_database:               default
+source_table:                  big_table
+destination_database:          default
+destination_table:             replicated_big_destination
+create_time:                   2025-12-15 13:12:48
+part_name:                     2025_0_32_3
+destination_file_paths:        ['replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.1.parquet','replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.2.parquet','replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.3.parquet','replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.4.parquet']
+elapsed:                       14.360427274
+rows_read:                     10256384 -- 10.26 million
+total_rows_to_read:            10485760 -- 10.49 million
+total_size_bytes_compressed:   83779395 -- 83.78 million
+total_size_bytes_uncompressed: 10611691600 -- 10.61 billion
+bytes_read_uncompressed:       10440998912 -- 10.44 billion
+memory_usage:                  89795477 -- 89.80 million
+peak_memory_usage:             107362133 -- 107.36 million
+
+1 row in set. Elapsed: 0.014 sec. 
+
+arthur :) select * from system.part_log where event_type = 'ExportPart' order by event_time desc limit 1 format Vertical;
+
+SELECT *
+FROM system.part_log
+WHERE event_type = 'ExportPart'
+ORDER BY event_time DESC
+LIMIT 1
+FORMAT Vertical
+
+Query id: 95128b01-b751-4726-8e3e-320728ac6af7
+
+Row 1:
+──────
+hostname:                arthur
+query_id:                
+event_type:              ExportPart
+merge_reason:            NotAMerge
+merge_algorithm:         Undecided
+event_date:              2025-12-15
+event_time:              2025-12-15 13:13:03
+event_time_microseconds: 2025-12-15 13:13:03.197492
+duration_ms:             14673
+database:                default
+table:                   big_table
+table_uuid:              a3eeeea0-295c-41a3-84ef-6b5463dbbe8c
+part_name:               2025_0_32_3
+partition_id:            2025
+partition:               2025
+part_type:               Wide
+disk_name:               default
+path_on_disk:            ./store/a3e/a3eeeea0-295c-41a3-84ef-6b5463dbbe8c/2025_0_32_3/
+remote_file_paths:       ['replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.1.parquet','replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.2.parquet','replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.3.parquet','replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.4.parquet']
+rows:                    10485760 -- 10.49 million
+size_in_bytes:           83779395 -- 83.78 million
+merged_from:             ['2025_0_32_3']
+bytes_uncompressed:      10611691600 -- 10.61 billion
+read_rows:               10485760 -- 10.49 million
+read_bytes:              10674503680 -- 10.67 billion
+peak_memory_usage:       107362133 -- 107.36 million
+error:                   0
+exception:               
+ProfileEvents:           {}
+
+1 row in set. Elapsed: 0.044 sec.
+
+arthur :) select _path, formatReadableSize(_size) as _size from s3(s3_conn, filename='**', format=One);
+
+SELECT
+    _path,
+    formatReadableSize(_size) AS _size
+FROM s3(s3_conn, filename = '**', format = One)
+
+Query id: c48ae709-f590-4d1b-8158-191f8d628966
+
+   ┌─_path────────────────────────────────────────────────────────────────────────────────┬─_size─────┐
+1. │ test/replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.1.parquet │ 17.36 MiB │
+2. │ test/replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.2.parquet │ 17.32 MiB │
+3. │ test/replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.4.parquet │ 5.04 MiB  │
+4. │ test/replicated_big/year=2025/2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7.3.parquet │ 17.40 MiB │
+5. │ test/replicated_big/year=2025/commit_2025_0_32_3_E439C23833C39C6E5104F6F4D1048BE7    │ 320.00 B  │
+   └──────────────────────────────────────────────────────────────────────────────────────┴───────────┘
+
+5 rows in set. Elapsed: 0.072 sec. 
+```