Avoid Nullable Columns

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

Note

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