I went over the documentation for Clickhouse and I did not see the option to UPDATE nor DELETE. It seems to me its an append only system. Is there a possibility to update existing records or is there some workaround like truncating a partition that has records in it that have changed and then re-insering the entire data for that partition?
11
votes
5 Answers
18
votes
ClickHouse doesn't support real UPDATE/DELETE. But there are few possible workarounds:
Trying to organize data in a way, that is need not to be updated. You could write log of update events to a table, and then calculate reports from that log. So, instead of updating existing records, you append new records to a table.
Using table engine that do data transformation in background during merges. For example, (rather specific) CollapsingMergeTree table engine: https://clickhouse.yandex/reference_en.html#CollapsingMergeTree Also there are ReplacingMergeTree table engine (not documented yet, you could find example in tests: https://github.com/yandex/ClickHouse/blob/master/dbms/tests/queries/0_stateless/00325_replacing_merge_tree.sql) Drawback is that you don't know, when background merge will be done, and will it ever be done.
Also look at samdoj's answer.
10
votes
Through Alter query in clickhouse we can able to delete/update the rows in a table.
For delete: Query should be constructed as
ALTER TABLE testing.Employee DELETE WHERE Emp_Name='user4';
For Update: Query should be constructed as
ALTER TABLE testing.employee UPDATE AssignedUser='sunil' where AssignedUser='sunny';
5
votes
You can drop and create new tables, but depending on their size this might be very time consuming. You could do something like this:
For deletion, something like this could work.
INSERT INTO tableTemp SELECT * from table1 WHERE rowID != @targetRowID;
DROP table1;
INSERT INTO table1 SELECT * from tableTemp;
Similarly, to update a row, you could first delete it in this manner, and then add it.
3
votes
Functionality to UPDATE or DELETE data has been added in recent ClickHouse releases, but its expensive batch operation which can't be performed too frequently.
See https://clickhouse.yandex/docs/en/query_language/alter/#mutations for more details.
