suneet-s commented on a change in pull request #9704: Refresh query docs.
URL: https://github.com/apache/druid/pull/9704#discussion_r408994814
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File path: docs/querying/datasource.md
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@@ -22,43 +22,317 @@ title: "Datasources"
~ under the License.
-->
+Datasources in Apache Druid are things that you can query. The most common
kind of datasource is a table datasource,
+and in many contexts the word "datasource" implicitly refers to table
datasources. This is especially true
+[during data ingestion](../ingestion/index.html), where ingestion is always
creating or writing into a table
+datasource. But at query time, there are many other types of datasources
available.
-A data source is the Apache Druid equivalent of a database table. However, a
query can also masquerade as a data source, providing subquery-like
functionality. Query data sources are currently supported only by
[GroupBy](../querying/groupbyquery.md) queries.
+In the [Druid SQL](sql.html) language, datasources are provided in the [`FROM`
clause](sql.html#from).
-### Table datasource
-The table data source is the most common type. It's represented by a string,
or by the full structure:
+The word "datasource" is generally spelled `dataSource` (with a capital S)
when it appears in API requests and
+responses.
+## Datasource type
+
+### `table`
+
+<!--DOCUSAURUS_CODE_TABS-->
+<!--SQL-->
+```sql
+SELECT column1, column2 FROM "druid"."dataSourceName"
+```
+<!--Native-->
+```json
+{
+ "queryType": "scan",
+ "dataSource": "dataSourceName",
+ "columns": ["column1", "column2"],
+ "intervals": ["0000/3000"]
+}
+```
+<!--END_DOCUSAURUS_CODE_TABS-->
+
+The table datasource is the most common type. This is the kind of datasource
you get when you perform
+[data ingestion](../ingestion/index.html). They are split up into segments,
distributed around the cluster,
+and queried in parallel.
+
+In [Druid SQL](sql.html#from), table datasources reside in the the `druid`
schema. This is the default schema, so table
+datasources can be referenced as either `druid.dataSourceName` or simply
`dataSourceName`.
+
+In native queries, table datasources can be referenced using their names as
strings (as in the example above), or by
+using JSON objects of the form:
+
+```json
+"dataSource": {
+ "type": "table",
+ "name": "dataSourceName"
+}
+```
+
+To see a list of all table datasources, use the SQL query
+`SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'druid'`.
+
+### `lookup`
+
+<!--DOCUSAURUS_CODE_TABS-->
+<!--SQL-->
+```sql
+SELECT k, v FROM lookup.countries
+```
+<!--Native-->
+```json
+{
+ "queryType": "scan",
+ "dataSource": {
+ "type": "lookup",
+ "lookup": "countries"
+ },
+ "columns": ["k", "v"],
+ "intervals": ["0000/3000"]
+}
+```
+<!--END_DOCUSAURUS_CODE_TABS-->
+
+Lookup datasources correspond to Druid's key-value [lookup](lookups.html)
objects. In [Druid SQL](sql.html#from),
+they reside in the the `lookup` schema. They are preloaded in memory on all
servers, so they can be accessed rapidly.
Review comment:
nit: all servers is vague. Are they pre-loaded on the master nodes too? I
think it can be configured so that only the nodes that need lookups have them
pre-loaded. My understanding is - brokers (always), historicals (always?),
overlord, MM/ Indexer (only if it's needed by ingestion).
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