CoalescingMergeTree
The engine inherits from MergeTree. The difference is that when merging data parts for CoalescingMergeTree tables ClickHouse replaces all the rows with the same primary key (or more accurately, with the same sorting key) with one row which contains the latest non-null values of each column. If the sorting key is composed in a way that a single key value corresponds to large number of rows, this significantly reduces storage volume and speeds up data selection.
We recommend using the engine together with MergeTree. Store complete data in MergeTree table, and use CoalescingMergeTree for aggregated data storing, for example, when preparing reports. Such an approach will prevent you from losing valuable data due to an incorrectly composed primary key.
Creating a Table
For a description of request parameters, see request description.
Parameters of CoalescingMergeTree
columns
columns - a tuple with the names of columns where values will be united. Optional parameter.
The columns must be of a numeric type and must not be in the partition or sorting key.
If columns is not specified, ClickHouse unites the values in all columns that are not in the sorting key.
Query clauses
When creating a CoalescingMergeTree table the same clauses are required, as when creating a MergeTree table.
Deprecated Method for Creating a Table
Do not use this method in new projects and, if possible, switch the old projects to the method described above.
All of the parameters excepting columns have the same meaning as in MergeTree.
columns— tuple with names of columns values of which will be summed. Optional parameter. For a description, see the text above.
Usage Example
Consider the following table:
Insert data to it:
The result will looks like this:
Recommended query for correct and deterministic result:
Using the FINAL modifier forces ClickHouse to apply merge logic at query time, ensuring you get the correct, coalesced "latest" value for each column. This is the safest and most accurate method when querying from a CoalescingMergeTree table.
An approach with GROUP BY may return incorrect results if the underlying parts have not been fully merged.
