Converting MergeTree to Replicated
Options here are:
- Use
INSERT INTO foo_replicated SELECT * FROM foo. (suitable for small tables) - Create table aside and attach all partition from the existing table then drop original table (uses hard links don’t require extra disk space).
ALTER TABLE foo_replicated ATTACH PARTITION ID 'bar' FROM 'foo'You can easily auto generate those commands using a query like:SELECT DISTINCT 'ALTER TABLE foo_replicated ATTACH PARTITION ID \'' || partition_id || '\' FROM foo;' from system.parts WHERE table = 'foo'; - Do it ‘in place’ using some file manipulation. see the procedure described here: https://clickhouse.tech/docs/en/engines/table-engines/mergetree-family/replication/#converting-from-mergetree-to-replicatedmergetree
- Do a backup of MergeTree and recover as ReplicatedMergeTree. https://github.com/Altinity/clickhouse-backup/blob/master/Examples.md#how-to-convert-mergetree-to-replicatedmegretree
- Embedded command for that should be added in future.
example for option 2
Note: ATTACH PARTITION ID ‘bar’ FROM ‘foo’` is practically free from compute and disk space perspective. This feature utilizes filesystem hard-links and the fact that files are immutable in ClickHouse® ( it’s the core of the ClickHouse design, filesystem hard-links and such file manipulations are widely used ).
create table foo( A Int64, D Date, S String )
Engine MergeTree
partition by toYYYYMM(D) order by A;
insert into foo select number, today(), '' from numbers(1e8);
insert into foo select number, today()-60, '' from numbers(1e8);
select count() from foo;
┌───count()─┐
│ 200000000 │
└───────────┘
create table foo_replicated as foo
Engine ReplicatedMergeTree('/clickhouse/{cluster}/tables/{database}/{table}/{shard}','{replica}')
partition by toYYYYMM(D) order by A;
SYSTEM STOP MERGES;
SELECT DISTINCT 'ALTER TABLE foo_replicated ATTACH PARTITION ID \'' || partition_id || '\' FROM foo;' from system.parts WHERE table = 'foo' AND active;
┌─concat('ALTER TABLE foo_replicated ATTACH PARTITION ID \'', partition_id, '\' FROM foo;')─┐
│ ALTER TABLE foo_replicated ATTACH PARTITION ID '202111' FROM foo; │
│ ALTER TABLE foo_replicated ATTACH PARTITION ID '202201' FROM foo; │
└───────────────────────────────────────────────────────────────────────────────────────────┘
clickhouse-client -q "SELECT DISTINCT 'ALTER TABLE foo_replicated ATTACH PARTITION ID \'' || partition_id || '\' FROM foo;' from system.parts WHERE table = 'foo' format TabSeparatedRaw" |clickhouse-client -mn
SYSTEM START MERGES;
SELECT count() FROM foo_replicated;
┌───count()─┐
│ 200000000 │
└───────────┘
rename table foo to foo_old, foo_replicated to foo;
-- you can drop foo_old any time later, it's kinda a cheap backup,
-- it cost nothing until you insert a lot of additional data into foo_replicated