Deduplicating Inserts on Retries

Insert operations can sometimes fail due to errors such as timeouts. When inserts fail, data may or may not have been successfully inserted. This guide covers how to enable deduplication on insert retries such that the same data does not get inserted more than once.

When an insert is retried, ClickHouse tries to determine whether the data has already been successfully inserted. If the inserted data is marked as a duplicate, ClickHouse does not insert it into the destination table. However, the user will still receive a successful operation status as if the data had been inserted normally.

Enabling insert deduplication on retries

Insert deduplication for tables

Only *MergeTree engines support deduplication on insertion.

For *ReplicatedMergeTree engines, insert deduplication is enabled by default and is controlled by the replicated_deduplication_window and replicated_deduplication_window_seconds settings. For non-replicated *MergeTree engines, deduplication is controlled by the non_replicated_deduplication_window setting.

The settings above determine the parameters of the deduplication log for a table. The deduplication log stores a finite number of block_ids, which determine how deduplication works (see below).

Query-level insert deduplication

The setting insert_deduplicate=1 enables deduplication at the query level. Note that if you insert data with insert_deduplicate=0, that data cannot be deduplicated even if you retry an insert with insert_deduplicate=1. This is because the block_ids are not written for blocks during inserts with insert_deduplicate=0.

How insert deduplication works

When data is inserted into ClickHouse, it is split data into blocks based on the number of rows and bytes.

For tables using *MergeTree engines, each block is assigned a unique block_id, which is a hash of the data in that block. This block_id is used as a unique key for the insert operation. If the same block_id is found in the deduplication log, the block is considered a duplicate and is not inserted into the table.

This approach works well for cases where inserts contain different data. However, if the same data is inserted multiple times intentionally, you need to use the insert_deduplication_token setting to control the deduplication process. This setting allows you to specify a unique token for each insert, which ClickHouse uses to determine whether the data is a duplicate.

For INSERT ... VALUES queries, splitting the inserted data into blocks is deterministic and is determined by settings. Therefore, users should retry insertions with the same settings values as the initial operation.

For INSERT ... SELECT queries, it is important that the SELECT part of the query returns the same data in the same order for each operation. Note that this is hard to achieve in practical usage. To ensure stable data order on retries, define a precise ORDER BY section in the SELECT part of the query. Keep in mind that it is possible that the selected table could be updated between retries: the result data could have changed and deduplication will not occur. Additionally, in situations where you are inserting large amounts of data, it is possible that the number of blocks after inserts can overflow the deduplication log window, and ClickHouse won't know to deduplicate the blocks.

Insert deduplication with materialized views

When a table has one or more materialized views, the inserted data is also inserted into the destination of those views with the defined transformations. The transformed data is also deduplicated on retries. ClickHouse performs deduplications for materialized views in the same way it deduplicates data inserted into the target table.

You can control this process using the following settings for the source table:

replicated_deduplication_window
replicated_deduplication_window_seconds
non_replicated_deduplication_window

You can also use the user profile setting deduplicate_blocks_in_dependent_materialized_views.

When inserting blocks into tables under materialized views, ClickHouse calculates the block_id by hashing a string that combines the block_ids from the source table and additional identifiers. This ensures accurate deduplication within materialized views, allowing data to be distinguished based on its original insertion, regardless of any transformations applied before reaching the destination table under the materialized view.

Examples

Identical blocks after materialized view transformations

Identical blocks, which have been generated during transformation inside a materialized view, are not deduplicated because they are based on different inserted data.

Here is an example:

CREATE TABLE dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000;

CREATE MATERIALIZED VIEW mv_dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000
AS SELECT
    0 AS key,
    value AS value
FROM dst;

SET max_block_size=1;
SET min_insert_block_size_rows=0;
SET min_insert_block_size_bytes=0;

The settings above allow us to select from a table with a series of blocks containing only one row. These small blocks are not squashed and remain the same until they are inserted into a table.

SET deduplicate_blocks_in_dependent_materialized_views=1;

We need to enable deduplication in materialized view:

INSERT INTO dst SELECT
    number + 1 AS key,
    IF(key = 0, 'A', 'B') AS value
FROM numbers(2);

SELECT
    *,
    _part
FROM dst
ORDER by all;

Here we see that two parts have been inserted into the dst table. 2 blocks from select -- 2 parts on insert. The parts contains different data.

SELECT
    *,
    _part
FROM mv_dst
ORDER by all;

Here we see that 2 parts have been inserted into the mv_dst table. That parts contain the same data, however they are not deduplicated.

INSERT INTO dst SELECT
    number + 1 AS key,
    IF(key = 0, 'A', 'B') AS value
FROM numbers(2);

SELECT
    *,
    _part
FROM dst
ORDER by all;

SELECT
    *,
    _part
FROM mv_dst
ORDER by all;

Here we see that when we retry the inserts, all data is deduplicated. Deduplication works for both the dst and mv_dst tables.

Identical blocks on insertion

CREATE TABLE dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000;


SET max_block_size=1;
SET min_insert_block_size_rows=0;
SET min_insert_block_size_bytes=0;

Insertion:

INSERT INTO dst SELECT
    0 AS key,
    'A' AS value
FROM numbers(2);

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

With the settings above, two blocks result from select– as a result, there should be two blocks for insertion into table dst. However, we see that only one block has been inserted into table dst. This occurred because second block has been deduplicated. It has the same data and the key for deduplication block_id which is calculated as a hash from the inserted data. This behaviour is not what was expected. Such cases are a rare occurence, but theoretically is possible. In order to handle such cases correctly, the user has to provide a insert_deduplication_token. Let's fix this with the following examples:

Identical blocks in insertion with `insert_deduplication_token`

CREATE TABLE dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000;

SET max_block_size=1;
SET min_insert_block_size_rows=0;
SET min_insert_block_size_bytes=0;

Insertion:

INSERT INTO dst SELECT
    0 AS key,
    'A' AS value
FROM numbers(2)
SETTINGS insert_deduplication_token='some_user_token';

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

Two identical blocks have been inserted as expected.

select 'second attempt';

INSERT INTO dst SELECT
    0 AS key,
    'A' AS value
FROM numbers(2)
SETTINGS insert_deduplication_token='some_user_token';

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

Retried insertion is deduplicated as expected.

select 'third attempt';

INSERT INTO dst SELECT
    1 AS key,
    'b' AS value
FROM numbers(2)
SETTINGS insert_deduplication_token='some_user_token';

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

That insertion is also deduplicated even though it contains different inserted data. Note that insert_deduplication_token has higher priority: ClickHouse does not use the hash sum of data when insert_deduplication_token is provided.

Different insert operations generate the same data after transformation in the underlying table of the materialized view

CREATE TABLE dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000;

CREATE MATERIALIZED VIEW mv_dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000
AS SELECT
    0 AS key,
    value AS value
FROM dst;

SET deduplicate_blocks_in_dependent_materialized_views=1;

select 'first attempt';

INSERT INTO dst VALUES (1, 'A');

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

SELECT
    'from mv_dst',
    *,
    _part
FROM mv_dst
ORDER by all;

select 'second attempt';

INSERT INTO dst VALUES (2, 'A');

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

SELECT
    'from mv_dst',
    *,
    _part
FROM mv_dst
ORDER by all;

We insert different data each time. However, the same data is inserted into the mv_dst table. Data is not deduplicated because the source data was different.

Different materialized view inserts into one underlying table with equivelant data

CREATE TABLE dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000;

CREATE TABLE mv_dst
(
    `key` Int64,
    `value` String
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS non_replicated_deduplication_window=1000;

CREATE MATERIALIZED VIEW mv_first
TO mv_dst
AS SELECT
    0 AS key,
    value AS value
FROM dst;

CREATE MATERIALIZED VIEW mv_second
TO mv_dst
AS SELECT
    0 AS key,
    value AS value
FROM dst;

SET deduplicate_blocks_in_dependent_materialized_views=1;

select 'first attempt';

INSERT INTO dst VALUES (1, 'A');

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

SELECT
    'from mv_dst',
    *,
    _part
FROM mv_dst
ORDER by all;

Two equal blocks inserted to the table mv_dst (as expected).

select 'second attempt';

INSERT INTO dst VALUES (1, 'A');

SELECT
    'from dst',
    *,
    _part
FROM dst
ORDER by all;

SELECT
    'from mv_dst',
    *,
    _part
FROM mv_dst
ORDER by all;

That retry operation is deduplicated on both tables dst and mv_dst.

Enabling insert deduplication on retries​

Insert deduplication for tables​

Query-level insert deduplication​

How insert deduplication works​

Insert deduplication with materialized views​

Examples​

Identical blocks after materialized view transformations​

Identical blocks on insertion​

Identical blocks in insertion with insert_deduplication_token​

Different insert operations generate the same data after transformation in the underlying table of the materialized view​

Different materialized view inserts into one underlying table with equivelant data​