Avoid nullable columns - ClickHouse Documentation

Nullable column (e.g. Nullable(String)) creates a separate column of UInt8 type. This additional column has to be processed every time a user works with a Nullable column. This leads to additional storage space used and almost always negatively affects performance. To avoid Nullable columns, consider setting a default value for that column. For example, instead of:

CREATE TABLE default.sample
(
    `x` Int8,
    `y` Nullable(Int8)
)
ENGINE = MergeTree
ORDER BY x

use

CREATE TABLE default.sample2
(
    `x` Int8,
    `y` Int8 DEFAULT 0
)
ENGINE = MergeTree
ORDER BY x

Consider your use case; a default value may be inappropriate.

Last modified on June 19, 2026

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