New type of MergeTree, CoalescingMergeTree -

Why it matters

This feature addresses the need to efficiently handle columns with nullable data by merging and coalescing multiple rows into a single representative row, taking the first non-Null value. It simplifies data consolidation and improves query accuracy on datasets where null values are common.

How to use it

Create a table using the CoalescingMergeTree engine in place of other MergeTree variants. The engine automatically coalesces nullable columns during background merges without additional configuration. For example:

CREATE TABLE example_table (
    id UInt64,
    value Nullable(String),
    date Date
) ENGINE = CoalescingMergeTree(date, (id), 8192);