Default type of the rule is removal ( DELETE). Type of the rule DELETE|TO DISK 'xxx'|TO VOLUME 'xxx'|GROUP BY specifies an action to be done with the part if the expression is satisfied (reaches current time): removal of expired rows, moving a part (if expression is satisfied for all rows in a part) to specified disk ( TO DISK 'xxx') or to volume ( TO VOLUME 'xxx'), or aggregating values in expired rows. Optional.Įxpression must have one Date or DateTime column as a result. TTL - A list of rules specifying storage duration of rows and defining logic of automatic parts movement between disks and volumes. Example: SAMPLE BY intHash32(UserID) ORDER BY (CounterID, EventDate, intHash32(UserID)). The result of a sampling expression must be an unsigned integer. If a sampling expression is used, the primary key must contain it. Thus in most cases it is unnecessary to specify a separate PRIMARY KEY clause. Optional.īy default the primary key is the same as the sorting key (which is specified by the ORDER BY clause). PRIMARY KEY - The primary key if it differs from the sorting key. The partition names here have the "YYYYMM" format. Don't partition your data by client identifiers or names (instead make client identifier or name the first column in the ORDER BY expression).įor partitioning by month, use the toYYYYMM(date_column) expression, where date_column is a column with a date of the type Date. You should never use too granular partitioning. Partitioning does not speed up queries (in contrast to the ORDER BY expression). In most cases you don't need partition key, and in most other cases you don't need partition key more granular than by months. Use the ORDER BY tuple() syntax, if you do not need sorting. Example: ORDER BY (CounterID, EventDate).ĬlickHouse uses the sorting key as a primary key if the primary key is not defined explicitly by the PRIMARY KEY clause. ORDER_BY Ī tuple of column names or arbitrary expressions. The MergeTree engine does not have parameters. Query Clauses ENGINE ĮNGINE - Name and parameters of the engine. If necessary, you can set the data sampling method in the table.įor a description of parameters, see the CREATE query description. For more information, see Data replication. The family of ReplicatedMergeTree tables provides data replication. ClickHouse also automatically cuts off the partition data where the partitioning key is specified in the query. Partitions can be used if the partitioning key is specified.ĬlickHouse supports certain operations with partitions that are more efficient than general operations on the same data with the same result. This allows you to create a small sparse index that helps find data faster. This method is much more efficient than continually rewriting the data in storage during insert. The data is quickly written to the table part by part, then rules are applied for merging the parts in the background. The MergeTree engine and other engines of this family ( *MergeTree) are the most robust ClickHouse table engines.Įngines in the MergeTree family are designed for inserting a very large amount of data into a table.
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