Why ClickHouse Silently Drops Your Rows I was running a batch test and noticed something strange. I inserted 3 rows and got back 2 when I ran SELECT count() . No error, no warning. ClickHouse just quietly dropped one. Turns out this is not a bug. It is the table engine doing exactly what it is designed to do. The Engine — ReplacingMergeTree When you create a table in ClickHouse, you pick an engine. The engine decides how data is stored and whether duplicates are kept or removed. ReplacingMergeTree is the engine that removes duplicates. You define an ORDER BY key and a version column. If two rows share the same ORDER BY key, only one survives — the one with the highest version value.…