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DOCS-271: Updated the MergeTree() ORDER BY description (#11433)
* CLICKHOUSEDOCS-271: Updated the MergeTree() ORDER BY description. * CLICKHOUSEDOCS-271: Fixes grammar. * CLICKHOUSEDOCS-271: Updated by comments. Co-authored-by: Sergei Shtykov <bayonet@yandex-team.ru>
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@ -41,8 +41,8 @@ CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
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INDEX index_name1 expr1 TYPE type1(...) GRANULARITY value1,
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INDEX index_name2 expr2 TYPE type2(...) GRANULARITY value2
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) ENGINE = MergeTree()
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ORDER BY expr
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[PARTITION BY expr]
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[ORDER BY expr]
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[PRIMARY KEY expr]
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[SAMPLE BY expr]
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[TTL expr [DELETE|TO DISK 'xxx'|TO VOLUME 'xxx'], ...]
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@ -58,23 +58,27 @@ For a description of parameters, see the [CREATE query description](../../../sql
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- `ENGINE` — Name and parameters of the engine. `ENGINE = MergeTree()`. The `MergeTree` engine does not have parameters.
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- `PARTITION BY` — The [partitioning key](custom-partitioning-key.md).
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- `ORDER BY` — The sorting key.
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A tuple of column names or arbitrary expressions. Example: `ORDER BY (CounterID, EventDate)`.
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ClickHouse uses the sorting key as a primary key if the primary key is not defined obviously by the `PRIMARY KEY` clause.
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Use the `ORDER BY tuple()` syntax, if you don't need sorting. See [Selecting the Primary Key](#selecting-the-primary-key).
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- `PARTITION BY` — The [partitioning key](custom-partitioning-key.md). Optional.
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For partitioning by month, use the `toYYYYMM(date_column)` expression, where `date_column` is a column with a date of the type [Date](../../../sql-reference/data-types/date.md). The partition names here have the `"YYYYMM"` format.
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- `ORDER BY` — The sorting key.
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A tuple of columns or arbitrary expressions. Example: `ORDER BY (CounterID, EventDate)`.
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- `PRIMARY KEY` — The primary key if it [differs from the sorting key](#choosing-a-primary-key-that-differs-from-the-sorting-key).
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- `PRIMARY KEY` — The primary key if it [differs from the sorting key](#choosing-a-primary-key-that-differs-from-the-sorting-key). Optional.
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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.
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- `SAMPLE BY` — An expression for sampling.
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- `SAMPLE BY` — An expression for sampling. Optional.
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If a sampling expression is used, the primary key must contain it. Example: `SAMPLE BY intHash32(UserID) ORDER BY (CounterID, EventDate, intHash32(UserID))`.
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- `TTL` — A list of rules specifying storage duration of rows and defining logic of automatic parts movement [between disks and volumes](#table_engine-mergetree-multiple-volumes).
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- `TTL` — A list of rules specifying storage duration of rows and defining logic of automatic parts movement [between disks and volumes](#table_engine-mergetree-multiple-volumes). Optional.
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Expression must have one `Date` or `DateTime` column as a result. Example:
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`TTL date + INTERVAL 1 DAY`
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@ -83,7 +87,7 @@ For a description of parameters, see the [CREATE query description](../../../sql
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For more details, see [TTL for columns and tables](#table_engine-mergetree-ttl)
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- `SETTINGS` — Additional parameters that control the behavior of the `MergeTree`:
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- `SETTINGS` — Additional parameters that control the behavior of the `MergeTree` (optional):
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- `index_granularity` — Maximum number of data rows between the marks of an index. Default value: 8192. See [Data Storage](#mergetree-data-storage).
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- `index_granularity_bytes` — Maximum size of data granules in bytes. Default value: 10Mb. To restrict the granule size only by number of rows, set to 0 (not recommended). See [Data Storage](#mergetree-data-storage).
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@ -198,6 +202,10 @@ The number of columns in the primary key is not explicitly limited. Depending on
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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.
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You can create a table without a primary key using the `ORDER BY tuple()` syntax. In this case, ClickHouse stores data in the order of inserting. If you want to save data order when inserting data by `INSERT ... SELECT` queries, set [max_insert_threads = 1](../../../operations/settings/settings.md#settings-max-insert-threads).
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To select data in the initial order, use [single-threaded](../../../operations/settings/settings.md#settings-max_threads) `SELECT` queries.
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### Choosing a Primary Key that Differs from the Sorting Key {#choosing-a-primary-key-that-differs-from-the-sorting-key}
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It is possible to specify a primary key (an expression with values that are written in the index file for each mark) that is different from the sorting key (an 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.
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@ -211,115 +211,26 @@ static String getMergeTreeVerboseHelp(bool is_extended_syntax)
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String help = R"(
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MergeTree is a family of storage engines.
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Syntax for the MergeTree table engine:
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MergeTrees are different in two ways:
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- they may be replicated and non-replicated;
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- they may do different actions on merge: nothing; sign collapse; sum; apply aggregete functions.
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CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
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(
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name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
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name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
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...
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INDEX index_name1 expr1 TYPE type1(...) GRANULARITY value1,
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INDEX index_name2 expr2 TYPE type2(...) GRANULARITY value2
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) ENGINE = MergeTree()
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ORDER BY expr
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[PARTITION BY expr]
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[PRIMARY KEY expr]
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[SAMPLE BY expr]
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[TTL expr [DELETE|TO DISK 'xxx'|TO VOLUME 'xxx'], ...]
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[SETTINGS name=value, ...]
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So we have 14 combinations:
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MergeTree, CollapsingMergeTree, SummingMergeTree, AggregatingMergeTree, ReplacingMergeTree, GraphiteMergeTree, VersionedCollapsingMergeTree
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ReplicatedMergeTree, ReplicatedCollapsingMergeTree, ReplicatedSummingMergeTree, ReplicatedAggregatingMergeTree, ReplicatedReplacingMergeTree, ReplicatedGraphiteMergeTree, ReplicatedVersionedCollapsingMergeTree
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See details in documentation: https://clickhouse.tech/docs/en/engines/table-engines/mergetree-family/mergetree/. Other engines of the family support different syntax, see details in the corresponding documentation topics.
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In most of cases, you need MergeTree or ReplicatedMergeTree.
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For replicated merge trees, you need to supply a path in ZooKeeper and a replica name as the first two parameters.
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Path in ZooKeeper is like '/clickhouse/tables/01/' where /clickhouse/tables/ is a common prefix and 01 is a shard name.
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Replica name is like 'mtstat01-1' - it may be the hostname or any suitable string identifying replica.
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You may use macro substitutions for these parameters. It's like ReplicatedMergeTree('/clickhouse/tables/{shard}/', '{replica}'...
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Look at the <macros> section in server configuration file.
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)";
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if (!is_extended_syntax)
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help += R"(
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Next parameter (which is the first for unreplicated tables and the third for replicated tables) is the name of date column.
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Date column must exist in the table and have type Date (not DateTime).
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It is used for internal data partitioning and works like some kind of index.
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If your source data doesn't have a column of type Date, but has a DateTime column, you may add values for Date column while loading,
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or you may INSERT your source data to a table of type Log and then transform it with INSERT INTO t SELECT toDate(time) AS date, * FROM ...
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If your source data doesn't have any date or time, you may just pass any constant for a date column while loading.
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Next parameter is optional sampling expression. Sampling expression is used to implement SAMPLE clause in query for approximate query execution.
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If you don't need approximate query execution, simply omit this parameter.
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Sample expression must be one of the elements of the primary key tuple. For example, if your primary key is (CounterID, EventDate, intHash64(UserID)), your sampling expression might be intHash64(UserID).
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Next parameter is the primary key tuple. It's like (CounterID, EventDate, intHash64(UserID)) - a list of column names or functional expressions in round brackets. If your primary key has just one element, you may omit round brackets.
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Careful choice of the primary key is extremely important for processing short-time queries.
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Next parameter is index (primary key) granularity. Good value is 8192. You have no reasons to use any other value.
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)";
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help += R"(
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For the Collapsing mode, the )" + (is_extended_syntax ? "only"s : "last"s) + R"( parameter is the name of a sign column - a special column that is used to 'collapse' rows with the same primary key while merging.
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For the Summing mode, the optional )" + (is_extended_syntax ? ""s : "last "s) + R"(parameter is a list of columns to sum while merging. This list is passed in round brackets, like (PageViews, Cost).
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If this parameter is omitted, the storage will sum all numeric columns except columns participating in the primary key.
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For the Replacing mode, the optional )" + (is_extended_syntax ? ""s : "last "s) + R"(parameter is the name of a 'version' column. While merging, for all rows with the same primary key, only one row is selected: the last row, if the version column was not specified, or the last row with the maximum version value, if specified.
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For VersionedCollapsing mode, the )" + (is_extended_syntax ? ""s : "last "s) + R"(2 parameters are the name of a sign column and the name of a 'version' column. Version column must be in primary key. While merging, a pair of rows with the same primary key and different sign may collapse.
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)";
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if (is_extended_syntax)
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help += R"(
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You can specify a partitioning expression in the PARTITION BY clause. It is optional but highly recommended.
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A common partitioning expression is some function of the event date column e.g. PARTITION BY toYYYYMM(EventDate) will partition the table by month.
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Rows with different partition expression values are never merged together. That allows manipulating partitions with ALTER commands.
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Also it acts as a kind of index.
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Sorting key is specified in the ORDER BY clause. It is mandatory for all MergeTree types.
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It is like (CounterID, EventDate, intHash64(UserID)) - a list of column names or functional expressions
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in round brackets.
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If your sorting key has just one element, you may omit round brackets.
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By default primary key is equal to the sorting key. You can specify a primary key that is a prefix of the
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sorting key in the PRIMARY KEY clause.
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Careful choice of the primary key is extremely important for processing short-time queries.
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Optional sampling expression can be specified in the SAMPLE BY clause. It is used to implement the SAMPLE clause in a SELECT query for approximate query execution.
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Sampling expression must be one of the elements of the primary key tuple. For example, if your primary key is (CounterID, EventDate, intHash64(UserID)), your sampling expression might be intHash64(UserID).
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Engine settings can be specified in the SETTINGS clause. Full list is in the source code in the 'src/Storages/MergeTree/MergeTreeSettings.h' file.
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E.g. you can specify the index (primary key) granularity with SETTINGS index_granularity = 8192.
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Examples:
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MergeTree PARTITION BY toYYYYMM(EventDate) ORDER BY (CounterID, EventDate) SETTINGS index_granularity = 8192
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MergeTree PARTITION BY toYYYYMM(EventDate) ORDER BY (CounterID, EventDate, intHash32(UserID), EventTime) SAMPLE BY intHash32(UserID)
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MergeTree PARTITION BY toYYYYMM(EventDate) ORDER BY (CounterID, EventDate, intHash32(UserID), EventTime) PRIMARY KEY (CounterID, EventDate) SAMPLE BY intHash32(UserID)
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CollapsingMergeTree(Sign) PARTITION BY StartDate SAMPLE BY intHash32(UserID) ORDER BY (CounterID, StartDate, intHash32(UserID), VisitID)
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SummingMergeTree PARTITION BY toMonday(EventDate) ORDER BY (OrderID, EventDate, BannerID, PhraseID, ContextType, RegionID, PageID, IsFlat, TypeID, ResourceNo)
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SummingMergeTree((Shows, Clicks, Cost, CostCur, ShowsSumPosition, ClicksSumPosition, SessionNum, SessionLen, SessionCost, GoalsNum, SessionDepth)) PARTITION BY toYYYYMM(EventDate) ORDER BY (OrderID, EventDate, BannerID, PhraseID, ContextType, RegionID, PageID, IsFlat, TypeID, ResourceNo)
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ReplicatedMergeTree('/clickhouse/tables/{layer}-{shard}/hits', '{replica}') PARTITION BY EventDate ORDER BY (CounterID, EventDate, intHash32(UserID), EventTime) SAMPLE BY intHash32(UserID)
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)";
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else
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help += R"(
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Examples:
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MergeTree(EventDate, (CounterID, EventDate), 8192)
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MergeTree(EventDate, intHash32(UserID), (CounterID, EventDate, intHash32(UserID), EventTime), 8192)
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CollapsingMergeTree(StartDate, intHash32(UserID), (CounterID, StartDate, intHash32(UserID), VisitID), 8192, Sign)
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SummingMergeTree(EventDate, (OrderID, EventDate, BannerID, PhraseID, ContextType, RegionID, PageID, IsFlat, TypeID, ResourceNo), 8192)
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SummingMergeTree(EventDate, (OrderID, EventDate, BannerID, PhraseID, ContextType, RegionID, PageID, IsFlat, TypeID, ResourceNo), 8192, (Shows, Clicks, Cost, CostCur, ShowsSumPosition, ClicksSumPosition, SessionNum, SessionLen, SessionCost, GoalsNum, SessionDepth))
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ReplicatedMergeTree('/clickhouse/tables/{layer}-{shard}/hits', '{replica}', EventDate, intHash32(UserID), (CounterID, EventDate, intHash32(UserID), EventTime), 8192)
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)";
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help += R"(
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For further info please read the documentation: https://clickhouse.yandex/
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If you use the Replicated version of engines, see https://clickhouse.tech/docs/en/engines/table-engines/mergetree-family/replication/.
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)";
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return help;
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