# Dimension Table

Dimension tables are a special kind of offline tables from which data can be looked up via the lookup UDF, providing a join like functionality. These dimension tables are replicated on all the hosts for a given tenant to allow faster lookups.

To mark an offline table as a dim table the configuration `isDimTable` should be set to true in the table config as shown below

```
{
  "OFFLINE": {
    "tableName": "dimBaseballTeams_OFFLINE",
    "tableType": "OFFLINE",
    "segmentsConfig": {
      "schemaName": "dimBaseballTeams",
    },
    "metadata": {},
    "quota": {
      "storage": "200M"
    },
    "isDimTable": true
  }
}
```

As dimension table are used to perform lookups of dimension values, they are required to have a primary key (can be a composite key).

```
{
  "dimensionFieldSpecs": [
    {
      "dataType": "STRING",
      "name": "teamID"
    },
    {
      "dataType": "STRING",
      "name": "teamName"
    }
  ],
  "schemaName": "dimBaseballTeams",
  "primaryKeyColumns": ["teamID"]
}
```

As mentioned above, when a table is marked as a dimension table it will be replicated on all the hosts, because of this the size of the dim table has to be small. The maximum size quota for a dimension table in a cluster is controlled by `controller.dimTable.maxSize` controller property. Table creation will fail if the storage quota exceeds this maximum size.


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