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There is a set of queries to change table settings. You can modify settings or reset them to default values. A single query can change several settings at once. If a setting with the specified name does not exist, then the query raises an exception. Syntax 
ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY|RESET SETTING ...
These queries can be applied to MergeTree tables only.

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MODIFY SETTING

Changes table settings. Syntax 
MODIFY SETTING setting_name=value [, ...]
Example 
CREATE TABLE example_table (id UInt32, data String) ENGINE=MergeTree() ORDER BY id;

ALTER TABLE example_table MODIFY SETTING max_part_loading_threads=8, max_parts_in_total=50000;

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RESET SETTING

Resets table settings to their default values. If a setting is in a default state, then no action is taken. Syntax 
RESET SETTING setting_name [, ...]
Example 
CREATE TABLE example_table (id UInt32, data String) ENGINE=MergeTree() ORDER BY id
    SETTINGS max_part_loading_threads=8;

ALTER TABLE example_table RESET SETTING max_part_loading_threads;
See Also