MergeTree¶

The MergeTree engine and other engines of this family (*MergeTree) are the most robust ClickHousе table engines.

The basic idea for MergeTree engines family is the following. When you have tremendous amount of a data that should be inserted into the table, you should write them quickly part by part and then merge parts by some rules in background. This method is much more efficient than constantly rewriting data in the storage at the insert.

Main features:

• Stores data sorted by primary key.

  This allows you to create a small sparse index that helps find data faster.

• This allows you to use partitions if the partitioning key is specified.

  ClickHouse supports certain operations with partitions that are more effective than general operations on the same data with the same result. ClickHouse also automatically cuts off the partition data where the partitioning key is specified in the query. This also increases the query performance.

• Data replication support.

  The family of ReplicatedMergeTree tables is used for this. For more information, see the Data replication section.

• Data sampling support.

  If necessary, you can set the data sampling method in the table.

Creating a Table¶

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
    INDEX index_name1 expr1 TYPE type1(...) GRANULARITY value1,
    INDEX index_name2 expr2 TYPE type2(...) GRANULARITY value2
) ENGINE = MergeTree()
[PARTITION BY expr]
[ORDER BY expr]
[PRIMARY KEY expr]
[SAMPLE BY expr]
[SETTINGS name=value, ...]

For a description of request parameters, see request description.

Query clauses

• ENGINE - Name and parameters of the engine. ENGINE = MergeTree(). MergeTree engine does not have parameters.

• PARTITION BY — The partitioning key.

  For partitioning by month, use the toYYYYMM(date_column) expression, where date_column is a column with a date of the type Date. The partition names here have the "YYYYMM" format.

• ORDER BY — The sorting key.

  A tuple of columns or arbitrary expressions. Example: ORDER BY (CounterID, EventDate).

• PRIMARY KEY - The primary key if it differs from the sorting key.

  By default the primary key is the same as the sorting key (which is specified by the ORDER BY clause). Thus in most cases it is unnecessary to specify a separate PRIMARY KEY clause.

• SAMPLE BY — An expression for sampling.

  If a sampling expression is used, the primary key must contain it. Example:
  SAMPLE BY intHash32(UserID) ORDER BY (CounterID, EventDate, intHash32(UserID)).

• SETTINGS — Additional parameters that control the behavior of the MergeTree:

  • index_granularity — The granularity of an index. The number of data rows between the "marks" of an index. By default, 8192. The list of all available parameters you can see in MergeTreeSettings.h.
  • min_merge_bytes_to_use_direct_io — The minimum data volume for merge operation required for using of the direct I/O access to the storage disk. During the merging of the data parts, ClickHouse calculates summary storage volume of all the data to be merged. If the volume exceeds min_merge_bytes_to_use_direct_io bytes, thеn ClickHouse reads and writes the data using direct I/O interface (O_DIRECT option) to the storage disk. If min_merge_bytes_to_use_direct_io = 0, then the direct I/O is disabled. Default value: 10 * 1024 * 1024 * 1024 bytes.

Example of sections setting

ENGINE MergeTree() PARTITION BY toYYYYMM(EventDate) ORDER BY (CounterID, EventDate, intHash32(UserID)) SAMPLE BY intHash32(UserID) SETTINGS index_granularity=8192

In the example, we set partitioning by month.

We also set an expression for sampling as a hash by the user ID. This allows you to pseudorandomize the data in the table for each CounterID and EventDate. If, when selecting the data, you define a SAMPLE clause, ClickHouse will return an evenly pseudorandom data sample for a subset of users.

index_granularity could be omitted because 8192 is the default value.

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
) ENGINE [=] MergeTree(date-column [, sampling_expression], (primary, key), index_granularity)

MergeTree(EventDate, intHash32(UserID), (CounterID, EventDate, intHash32(UserID)), 8192)

Data Storage¶

A table consists of data parts sorted by primary key.

When data is inserted in a table, separate data parts are created and each of them is lexicographically sorted by primary key. For example, if the primary key is (CounterID, Date), the data in the part is sorted by CounterID, and within each CounterID, it is ordered by Date.

Data belonging to different partitions are separated into different parts. In the background, ClickHouse merges data parts for more efficient storage. Parts belonging to different partitions are not merged. The merge mechanism does not guarantee that all rows with the same primary key will be in the same data part.

For each data part, ClickHouse creates an index file that contains the primary key value for each index row ("mark"). Index row numbers are defined as n * index_granularity. The maximum value n is equal to the integer part of dividing the total number of rows by the index_granularity. For each column, the "marks" are also written for the same index rows as the primary key. These "marks" allow you to find the data directly in the columns.

You can use a single large table and continually add data to it in small chunks – this is what the MergeTree engine is intended for.

Primary Keys and Indexes in Queries¶

Let's take the (CounterID, Date) primary key. In this case, the sorting and index can be illustrated as follows:

Whole data:     [-------------------------------------------------------------------------]
CounterID:      [aaaaaaaaaaaaaaaaaabbbbcdeeeeeeeeeeeeefgggggggghhhhhhhhhiiiiiiiiikllllllll]
Date:           [1111111222222233331233211111222222333211111112122222223111112223311122333]
Marks:           |      |      |      |      |      |      |      |      |      |      |
                a,1    a,2    a,3    b,3    e,2    e,3    g,1    h,2    i,1    i,3    l,3
Marks numbers:   0      1      2      3      4      5      6      7      8      9      10

If the data query specifies:

• CounterID in ('a', 'h'), the server reads the data in the ranges of marks [0, 3) and [6, 8).
• CounterID IN ('a', 'h') AND Date = 3, the server reads the data in the ranges of marks [1, 3) and [7, 8).
• Date = 3, the server reads the data in the range of marks [1, 10].

The examples above show that it is always more effective to use an index than a full scan.

A sparse index allows extra strings to be read. When reading a single range of the primary key, up to index_granularity * 2 extra rows in each data block can be read. In most cases, ClickHouse performance does not degrade when index_granularity = 8192.

Sparse indexes allow you to work with a very large number of table rows, because such indexes are always stored in the computer's RAM.

ClickHouse does not require a unique primary key. You can insert multiple rows with the same primary key.

Selecting the Primary Key¶

The number of columns in the primary key is not explicitly limited. Depending on the data structure, you can include more or fewer columns in the primary key. This may:

• Improve the performance of an index.

  If the primary key is (a, b), then adding another column c will improve the performance if the following conditions are met: - There are queries with a condition on column c. - Long data ranges (several times longer than the index_granularity) with identical values for (a, b) are common. In other words, when adding another column allows you to skip quite long data ranges.

• Improve data compression.

  ClickHouse sorts data by primary key, so the higher the consistency, the better the compression.

• Provide additional logic when data parts merging in the CollapsingMergeTree and SummingMergeTree engines.

  In this case it makes sense to specify the sorting key that is different from the primary key.

A long primary key will negatively affect the insert performance and memory consumption, but extra columns in the primary key do not affect ClickHouse performance during SELECT queries.

Choosing the Primary Key that differs from the Sorting Key¶

It is possible to specify the primary key (the expression, values of which are written into the index file for each mark) that is different from the sorting key (the expression for sorting the rows in data parts). In this case the primary key expression tuple must be a prefix of the sorting key expression tuple.

This feature is helpful when using the SummingMergeTree and AggregatingMergeTree table engines. In a common case when using these engines the table has two types of columns: dimensions and measures. Typical queries aggregate values of measure columns with arbitrary GROUP BY and filtering by dimensions. As SummingMergeTree and AggregatingMergeTree aggregate rows with the same value of the sorting key, it is natural to add all dimensions to it. As a result the key expression consists of a long list of columns and this list must be frequently updated with newly added dimensions.

In this case it makes sense to leave only a few columns in the primary key that will provide efficient range scans and add the remaining dimension columns to the sorting key tuple.

ALTER of the sorting key is a lightweight operation because when a new column is simultaneously added to the table and to the sorting key data parts need not be changed (they remain sorted by the new sorting key expression).

Use of Indexes and Partitions in Queries¶

ForSELECT queries, ClickHouse analyzes whether an index can be used. An index can be used if the WHERE/PREWHERE clause has an expression (as one of the conjunction elements, or entirely) that represents an equality or inequality comparison operation, or if it has IN or LIKE with a fixed prefix on columns or expressions that are in the primary key or partitioning key, or on certain partially repetitive functions of these columns, or logical relationships of these expressions.

Thus, it is possible to quickly run queries on one or many ranges of the primary key. In this example, queries will be fast when run for a specific tracking tag; for a specific tag and date range; for a specific tag and date; for multiple tags with a date range, and so on.

Let's look at the engine configured as follows:

ENGINE MergeTree() PARTITION BY toYYYYMM(EventDate) ORDER BY (CounterID, EventDate) SETTINGS index_granularity=8192

In this case, in queries:

SELECT count() FROM table WHERE EventDate = toDate(now()) AND CounterID = 34
SELECT count() FROM table WHERE EventDate = toDate(now()) AND (CounterID = 34 OR CounterID = 42)
SELECT count() FROM table WHERE ((EventDate >= toDate('2014-01-01') AND EventDate <= toDate('2014-01-31')) OR EventDate = toDate('2014-05-01')) AND CounterID IN (101500, 731962, 160656) AND (CounterID = 101500 OR EventDate != toDate('2014-05-01'))

ClickHouse will use the primary key index to trim improper data and the monthly partitioning key to trim partitions that are in improper date ranges.

The queries above show that the index is used even for complex expressions. Reading from the table is organized so that using the index can't be slower than a full scan.

In the example below, the index can't be used.

SELECT count() FROM table WHERE CounterID = 34 OR URL LIKE '%upyachka%'

To check whether ClickHouse can use the index when running a query, use the settings force_index_by_date and force_primary_key.

The key for partitioning by month allows reading only those data blocks which contain dates from the proper range. In this case, the data block may contain data for many dates (up to an entire month). Within a block, data is sorted by primary key, which might not contain the date as the first column. Because of this, using a query with only a date condition that does not specify the primary key prefix will cause more data to be read than for a single date.

Data Skipping Indices (Experimental)¶

You need to set allow_experimental_data_skipping_indices to 1 to use indices. (run SET allow_experimental_data_skipping_indices = 1).

Index declaration in the columns section of the CREATE query.

INDEX index_name expr TYPE type(...) GRANULARITY granularity_value

For tables from the *MergeTree family data skipping indices can be specified.

These indices aggregate some information about the specified expression on blocks, which consist of granularity_value granules, then these aggregates are used in SELECT queries for reducing the amount of data to read from the disk by skipping big blocks of data where where query cannot be satisfied.

Example

CREATE TABLE table_name
(
    u64 UInt64,
    i32 Int32,
    s String,
    ...
    INDEX a (u64 * i32, s) TYPE minmax GRANULARITY 3,
    INDEX b (u64 * length(s)) TYPE set(1000) GRANULARITY 4
) ENGINE = MergeTree()
...

Indices from the example can be used by ClickHouse to reduce the amount of data to read from disk in following queries.

SELECT count() FROM table WHERE s < 'z'
SELECT count() FROM table WHERE u64 * i32 == 10 AND u64 * length(s) >= 1234

Available Types of Indices¶

• minmax Stores extremes of the specified expression (if the expression is tuple, then it stores extremes for each element of tuple), uses stored info for skipping blocks of the data like the primary key.

• set(max_rows) Stores unique values of the specified expression (no more than max_rows rows), use them to check if the WHERE expression is not satisfiable on a block of the data.

INDEX sample_index (u64 * length(s)) TYPE minmax GRANULARITY 4
INDEX b (u64 * length(str), i32 + f64 * 100, date, str) TYPE set(100) GRANULARITY 4

Concurrent Data Access¶

For concurrent table access, we use multi-versioning. In other words, when a table is simultaneously read and updated, data is read from a set of parts that is current at the time of the query. There are no lengthy locks. Inserts do not get in the way of read operations.

Reading from a table is automatically parallelized.

Original article