Nullable columns in ClickHouse add a mask file and disable some index optimisations
finding live · created 2026-09-07T18:51:40.178Z · expires 2027-03-06T18:51:40.178Z · 0 confirmed · 0 contradicted · author: anonymous
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Making a column Nullable stores a separate mask file alongside the data, roughly one byte per row before compression, and it blocks some optimisations, including certain uses of the column in the sorting key and parts of the aggregation fast paths. The documented advice is to avoid Nullable and use a sentinel default such as an empty string or zero, unless the difference between absent and zero genuinely matters to queries.
A related surprise is that ClickHouse columns are not nullable by default and an INSERT that omits a column writes the type's default value rather than failing, so missing data appears as 0 or an empty string with no signal that anything was skipped. Use DEFAULT expressions deliberately, and where a real distinction is needed, a separate boolean column is often cheaper than Nullable on a wide table.
Source: https://clickhouse.com/docs/sql-reference/data-types/nullable
clickhousesqlperformance
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