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Background

Partitioning is specified on a table when it is initially defined via the PARTITION BY clause. This clause can contain a SQL expression on any columns, the results of which will define which partition a row is sent to. The data parts are logically associated with each partition on disk and can be queried in isolation. For the example below, we partition the posts table by year using the expression toYear(CreationDate). As rows are inserted into ClickHouse, this expression will be evaluated against each row and routed to the resulting partition if it exists (if the row is the first for a year, the partition will be created). 
 CREATE TABLE posts
(
        `Id` Int32 CODEC(Delta(4), ZSTD(1)),
        `PostTypeId` Enum8('Question' = 1, 'Answer' = 2, 'Wiki' = 3, 'TagWikiExcerpt' = 4, 'TagWiki' = 5, 'ModeratorNomination' = 6, 'WikiPlaceholder' = 7, 'PrivilegeWiki' = 8),
        `AcceptedAnswerId` UInt32,
        `CreationDate` DateTime64(3, 'UTC'),
...
        `ClosedDate` DateTime64(3, 'UTC')
)
ENGINE = MergeTree
ORDER BY (PostTypeId, toDate(CreationDate), CreationDate)
PARTITION BY toYear(CreationDate)
Read about setting the partition expression in a section How to set the partition expression. In ClickHouse, users should principally consider partitioning to be a data management feature, not a query optimization technique. By separating data logically based on a key, each partition can be operated on independently e.g. deleted. This allows users to move partitions, and thus subsets, between storage tiers efficiently on time or expire data/efficiently delete from the cluster.

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Drop partitions

ALTER TABLE ... DROP PARTITION provides a cost-efficient way to drop a whole partition. 
ALTER TABLE table_name [ON CLUSTER cluster] DROP PARTITION|PART partition_expr
This query tags the partition as inactive and deletes data completely, approximately in 10 minutes. The query is replicated – it deletes data on all replicas. In example, below we remove posts from 2008 for the earlier table by dropping the associated partition. 
SELECT DISTINCT partition
FROM system.parts
WHERE `table` = 'posts'

┌─partition─┐
│ 2008      │
│ 2009      │
│ 2010      │
│ 2011      │
│ 2012      │
│ 2013      │
│ 2014      │
│ 2015      │
│ 2016      │
│ 2017      │
│ 2018      │
│ 2019      │
│ 2020      │
│ 2021      │
│ 2022      │
│ 2023      │
│ 2024      │
└───────────┘

17 rows in set. Elapsed: 0.002 sec.

ALTER TABLE posts
(DROP PARTITION '2008')

0 rows in set. Elapsed: 0.103 sec.