ClickHouse ReplacingMergeTree - Why Your Row Count Does Not Match

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. ReplacingMergeTr