# How a table is stored ## A table's Storage tab Double-click a ClickHouse table and, beside Properties, Telemetry and Data, there is **Storage**: how the table is actually stored, which on a MergeTree table is where its behaviour lives. - **The engine**, as the server wrote it: `ENGINE`, `PARTITION BY`, `ORDER BY`, `SAMPLE BY`, the `TTL` and the table's own `SETTINGS`. `PRIMARY KEY` appears only when the sparse index is shorter than the sort order, which is the only time it says anything the `ORDER BY` line did not. Beside it: rows, size on disk, the compression ratio, the active parts, the granules and the storage policy. - **Partitions**, largest first, each with its parts, rows, size on disk, size raw, the time range it covers and when it was last written. A partition is read and dropped as a unit, and its parts are what merges combine, so this is where "too many parts" is visible before an insert is refused. - **Projections**: the second copies of the table kept in another order or pre-aggregated, with what each holds and what it costs, and the query that defines it. - **Data-skipping indexes**, with the expression each summarises, how many granules each entry covers, and its size. ClickHouse has no B-tree, so for a filter on a column outside the sort key these are the only thing that can skip anything. Three actions sit on each partition row: **Merge now** (`OPTIMIZE ... PARTITION ... FINAL`, which merges its parts into one instead of waiting for the server to), **Detach** (the rows leave the table but the files stay on disk, so they can be attached back) and **Drop**. Each one shows you the statement it is about to run before it runs, each goes into your query history, and on a connection flagged production they are confirmed as production changes. On a read-only connection they are not offered at all. A table with no parts on disk (a Memory table, a Dictionary, a Distributed table) says so, rather than showing three empty lists. ## Designing a table The table designer has an **Engine** section for ClickHouse, because a MergeTree table's engine is not a detail you fill in afterwards. Pick from the MergeTree family, each with a line saying what it does to a row: **MergeTree** keeps every row; **ReplacingMergeTree** keeps the last row for each sort key; **SummingMergeTree** adds up the numeric columns of rows that share one; **AggregatingMergeTree** combines their `AggregateFunction` columns; **CollapsingMergeTree** lets a row be cancelled by writing it again with `sign = -1`; **VersionedCollapsingMergeTree** does the same when the rows can arrive out of order. Where the engine takes an argument (a version column, a sign column, the columns to sum) the field appears with it. Below that: **Partition by**, **TTL** and **Settings**, and in the Columns grid a **Codec** field per column, which on a columnar store is the difference between a column costing eight bytes a row and costing almost nothing. The **sort key** is shown but not typed here: it is the columns you tick as the key in Columns, so it is decided in one place. Two things are said before the statement runs rather than after the server refuses it: an engine missing an argument it needs, and a deduplicating engine with no sort key, which is legal and merges the whole table into a single row. If the server has replicas, **Create on every replica** writes the table `ON CLUSTER` with a `Replicated` engine, so every replica has it and they keep each other up to date. An existing table's engine, sort key and partitioning are the order its rows are written in and cannot be changed in place; the designer says so and points at the table's [Storage tab](#a-tables-storage-tab), which is at the top of this page. ## Schema sync and data flows Schema sync and data flows write ClickHouse DDL too: tables are copied from the server's own `CREATE TABLE`, so engine clauses survive.