## What does Atlan crawl from ClickHouse?

URL: https://docs.atlan.com/apps/connectors/database/clickhouse/references/what-does-atlan-crawl-from-clickhouse

> During a ClickHouse crawl, Atlan extracts metadata for Databases, Schemas, Tables, Views, and Columns. Reference tables map each ClickHouse property to its Atlan asset.

Atlan extracts metadata for Databases, Schemas, Tables, Views, and Columns from ClickHouse and maps each property to its internal asset model. The tables below list which ClickHouse properties are read and how they appear on the corresponding Atlan asset.

Once you have [crawled ClickHouse](https://docs.atlan.com/llms/connectors/clickhouse/crawl-clickhouse/llms.txt), you can [use connector-specific filters](https://docs.atlan.com/llms/catalog/discovery/use-the-filters-menu/llms.txt) for quick asset discovery.

## Databases

ClickHouse uses a single-level database hierarchy. Atlan maps the top-level catalog as `default` to its `Database` asset type, and maps each ClickHouse database to the `Schema` asset type.

| Source property | Atlan property | Where in Atlan |
| --- | --- | --- |
| `default` (catalog) | `name` | asset preview and profile, overview sidebar |
| `SCHEMA_COUNT` | `schemaCount` | asset filters |

## Schemas

Atlan maps ClickHouse databases to its `Schema` asset type.

| Source property | Atlan property | Where in Atlan |
| --- | --- | --- |
| `system.databases.name` | `name` | asset preview and profile, overview sidebar |
| `TABLE_COUNT` | `tableCount` | asset preview and profile |
| `VIEW_COUNT` | `viewsCount` | asset preview and profile |
| `default` (catalog) | `databaseName` | asset preview and profile, overview sidebar |

## Tables

Atlan maps tables from ClickHouse to its `Table` asset type.

| Source property | Atlan property | Where in Atlan |
| --- | --- | --- |
| `system.tables.name` | `name` | asset preview and profile, overview sidebar |
| `system.tables.engine` | `definition` (engine type) | asset profile, overview sidebar |
| `COLUMN_COUNT` | `columnCount` | asset preview and profile, overview sidebar |
| `system.tables.total_rows` | `rowCount` | asset preview and profile, overview sidebar |
| `system.tables.total_bytes` | `sizeBytes` | asset preview and profile, overview sidebar |
| `system.tables.partition_key` | `partitionStrategy` | API only |
| `system.tables.sorting_key` | custom metadata | API only |
| `system.tables.primary_key` | custom metadata | API only |

## Views

Atlan maps views from ClickHouse to its `View` asset type. This includes both regular views and materialized views.

| Source property | Atlan property | Where in Atlan |
| --- | --- | --- |
| `system.tables.name` | `name` | asset preview and profile, overview sidebar |
| `COLUMN_COUNT` | `columnCount` | asset preview and profile, overview sidebar |
| `system.tables.create_table_query` | `definition` | asset profile and overview sidebar |
| `system.tables.engine` | `subType` (VIEW or MATERIALIZED VIEW) | API only |

## Columns

Atlan maps columns from ClickHouse to its `Column` asset type.

| Source property | Atlan property | Where in Atlan |
| --- | --- | --- |
| `system.columns.name` | `name` | asset profile and filter, overview sidebar |
| `system.columns.comment` | `description` | asset profile and filter, overview sidebar |
| `system.columns.position` | `order` | parent asset profile |
| `system.columns.type` | `dataType` | asset preview and filter, overview sidebar, parent asset profile |
| `system.columns.is_in_primary_key` | `isPrimary` | asset preview and filter, overview sidebar, parent asset profile |
| `Nullable(...)` type detection | `isNullable` | API only |
| `system.columns.numeric_scale` | `numericScale` | API only |
| `system.columns.numeric_precision` | `precision` | API only |

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