druid/docs/querying/datasource.md

---
id: datasource
title: "Datasources"
---

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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.md), where ingestion is always creating or writing into a table
datasource. But at query time, there are many other types of datasources available.

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.md). They are split up into segments, distributed around the cluster,
and queried in parallel.

In [Druid SQL](sql.md#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.md) objects. In [Druid SQL](sql.md#from),
they reside in the the `lookup` schema. They are preloaded in memory on all servers, so they can be accessed rapidly.
They can be joined onto regular tables using the [join operator](#join).

Lookup datasources are key-value oriented and always have exactly two columns: `k` (the key) and `v` (the value), and
both are always strings.

To see a list of all lookup datasources, use the SQL query
`SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'lookup'`.

> Performance tip: Lookups can be joined with a base table either using an explicit [join](#join), or by using the
> SQL [`LOOKUP` function](sql.md#string-functions).
> However, the join operator must evaluate the condition on each row, whereas the
> `LOOKUP` function can defer evaluation until after an aggregation phase. This means that the `LOOKUP` function is
> usually faster than joining to a lookup datasource.

Refer to the [Query execution](query-execution.md#table) page for more details on how queries are executed when you
use table datasources.

### `union`

<!--DOCUSAURUS_CODE_TABS-->
<!--Native-->
```json
{
  "queryType": "scan",
  "dataSource": {
    "type": "union",
    "dataSources": ["<tableDataSourceName1>", "<tableDataSourceName2>", "<tableDataSourceName3>"]
  },
  "columns": ["column1", "column2"],
  "intervals": ["0000/3000"]
}
```
<!--END_DOCUSAURUS_CODE_TABS-->

Union datasources allow you to treat two or more table datasources as a single datasource. The datasources being unioned
do not need to have identical schemas. If they do not fully match up, then columns that exist in one table but not
another will be treated as if they contained all null values in the tables where they do not exist.

The list of "dataSources" must be nonempty. If you want to query an empty dataset, use an [`inline` datasource](#inline)
instead.

Union datasources are not available in Druid SQL.

Refer to the [Query execution](query-execution.md#union) page for more details on how queries are executed when you
use union datasources.

### `inline`

<!--DOCUSAURUS_CODE_TABS-->
<!--Native-->
```json
{
  "queryType": "scan",
  "dataSource": {
    "type": "inline",
    "columnNames": ["country", "city"],
    "rows": [
      ["United States", "San Francisco"],
      ["Canada", "Calgary"]
    ]
  },
  "columns": ["country", "city"],
  "intervals": ["0000/3000"]
}
```
<!--END_DOCUSAURUS_CODE_TABS-->

Inline datasources allow you to query a small amount of data that is embedded in the query itself. They are useful when
you want to write a query on a small amount of data without loading it first. They are also useful as inputs into a
[join](#join). Druid also uses them internally to handle subqueries that need to be inlined on the Broker. See the
[`query` datasource](#query) documentation for more details.

There are two fields in an inline datasource: an array of `columnNames` and an array of `rows`. Each row is an array
that must be exactly as long as the list of `columnNames`. The first element in each row corresponds to the first
column in `columnNames`, and so on.

Inline datasources are not available in Druid SQL.

Refer to the [Query execution](query-execution.md#inline) page for more details on how queries are executed when you
use inline datasources.

### `query`

<!--DOCUSAURUS_CODE_TABS-->
<!--SQL-->
```sql
-- Uses a subquery to count hits per page, then takes the average.
SELECT
  AVG(cnt) AS average_hits_per_page
FROM
  (SELECT page, COUNT(*) AS hits FROM site_traffic GROUP BY page)
```
<!--Native-->
```json
{
  "queryType": "timeseries",
  "dataSource": {
    "type": "query",
    "query": {
      "queryType": "groupBy",
      "dataSource": "site_traffic",
      "intervals": ["0000/3000"],
      "granularity": "all",
      "dimensions": ["page"],
      "aggregations": [
        { "type": "count", "name": "hits" }
      ]
    }
  },
  "intervals": ["0000/3000"],
  "granularity": "all",
  "aggregations": [
    { "type": "longSum", "name": "hits", "fieldName": "hits" },
    { "type": "count", "name": "pages" }
  ],
  "postAggregations": [
    { "type": "expression", "name": "average_hits_per_page", "expression": "hits / pages" }
  ]
}
```
<!--END_DOCUSAURUS_CODE_TABS-->

Query datasources allow you to issue subqueries. In native queries, they can appear anywhere that accepts a
`dataSource`. In SQL, they can appear in the following places, always surrounded by parentheses:

- The FROM clause: `FROM (<subquery>)`.
- As inputs to a JOIN: `<table-or-subquery-1> t1 INNER JOIN <table-or-subquery-2> t2 ON t1.<col1> = t2.<col2>`.
- In the WHERE clause: `WHERE <column> { IN | NOT IN } (<subquery>)`. These are translated to joins by the SQL planner.

> Performance tip: In most cases, subquery results are fully buffered in memory on the Broker and then further
> processing occurs on the Broker itself. This means that subqueries with large result sets can cause performance
> bottlenecks or run into memory usage limits on the Broker. See the [Query execution](query-execution.md#query)
> page for more details on how subqueries are executed and what limits will apply.

### `join`

<!--DOCUSAURUS_CODE_TABS-->
<!--SQL-->
```sql
-- Joins "sales" with "countries" (using "store" as the join key) to get sales by country.
SELECT
  store_to_country.v AS country,
  SUM(sales.revenue) AS country_revenue
FROM
  sales
  INNER JOIN lookup.store_to_country ON sales.store = store_to_country.k
GROUP BY
  countries.v
```
<!--Native-->
```json
{
  "queryType": "groupBy",
  "dataSource": {
    "type": "join",
    "left": "sales",
    "right": {
      "type": "lookup",
      "lookup": "store_to_country"
    },
    "rightPrefix": "r.",
    "condition": "store == \"r.k\"",
    "joinType": "INNER"
  },
  "intervals": ["0000/3000"],
  "granularity": "all",
  "dimensions": [
    { "type": "default", "outputName": "country", "dimension": "r.v" }
  ],
  "aggregations": [
    { "type": "longSum", "name": "country_revenue", "fieldName": "revenue" }
  ]
}
```
<!--END_DOCUSAURUS_CODE_TABS-->

Join datasources allow you to do a SQL-style join of two datasources. Stacking joins on top of each other allows
you to join arbitrarily many datasources.

In Druid {{DRUIDVERSION}}, joins are implemented with a broadcast hash-join algorithm. This means that all tables
other than the leftmost "base" table must fit in memory. It also means that the join condition must be an equality. This
feature is intended mainly to allow joining regular Druid tables with [lookup](#lookup), [inline](#inline), and
[query](#query) datasources.

Refer to the [Query execution](query-execution.md#join) page for more details on how queries are executed when you
use join datasources.

#### Joins in SQL

SQL joins take the form:

```
<o1> [ INNER | LEFT [OUTER] ] JOIN <o2> ON <condition>
```

The condition must involve only equalities, but functions are okay, and there can be multiple equalities ANDed together.
Conditions like `t1.x = t2.x`, or `LOWER(t1.x) = t2.x`, or `t1.x = t2.x AND t1.y = t2.y` can all be handled. Conditions
like `t1.x <> t2.x` cannot currently be handled.

Note that Druid SQL is less rigid than what native join datasources can handle. In cases where a SQL query does
something that is not allowed as-is with a native join datasource, Druid SQL will generate a subquery. This can have
a substantial effect on performance and scalability, so it is something to watch out for. Some examples of when the
SQL layer will generate subqueries include:

- Joining a regular Druid table to itself, or to another regular Druid table. The native join datasource can accept
a table on the left-hand side, but not the right, so a subquery is needed.

- Join conditions where the expressions on either side are of different types.

- Join conditions where the right-hand expression is not a direct column access.

For more information about how Druid translates SQL to native queries, refer to the
[Druid SQL](sql.md#query-translation) documentation.

#### Joins in native queries

Native join datasources have the following properties. All are required.

|Field|Description|
|-----|-----------|
|`left`|Left-hand datasource. Must be of type `table`, `join`, `lookup`, `query`, or `inline`. Placing another join as the left datasource allows you to join arbitrarily many datasources.|
|`right`|Right-hand datasource. Must be of type `lookup`, `query`, or `inline`. Note that this is more rigid than what Druid SQL requires.|
|`rightPrefix`|String prefix that will be applied to all columns from the right-hand datasource, to prevent them from colliding with columns from the left-hand datasource. Can be any string, so long as it is nonempty and is not be a prefix of the string `__time`. Any columns from the left-hand side that start with your `rightPrefix` will be shadowed. It is up to you to provide a prefix that will not shadow any important columns from the left side.|
|`condition`|[Expression](../misc/math-expr.md) that must be an equality where one side is an expression of the left-hand side, and the other side is a simple column reference to the right-hand side. Note that this is more rigid than what Druid SQL requires: here, the right-hand reference must be a simple column reference; in SQL it can be an expression.|
|`joinType`|`INNER` or `LEFT`.|

#### Join performance

Joins are a feature that can significantly affect performance of your queries. Some performance tips and notes:

1. Joins are especially useful with [lookup datasources](#lookup), but in most cases, the
[`LOOKUP` function](sql.md#string-functions) performs better than a join. Consider using the `LOOKUP` function if
it is appropriate for your use case.
2. When using joins in Druid SQL, keep in mind that it can generate subqueries that you did not explicitly include in
your queries. Refer to the [Druid SQL](sql.md#query-translation) documentation for more details about when this happens
and how to detect it.
3. One common reason for implicit subquery generation is if the types of the two halves of an equality do not match.
For example, since lookup keys are always strings, the condition `druid.d JOIN lookup.l ON d.field = l.field` will
perform best if `d.field` is a string.
4. As of Druid {{DRUIDVERSION}}, the join operator must evaluate the condition for each row. In the future, we expect
to implement both early and deferred condition evaluation, which we expect to improve performance considerably for
common use cases.
5. Currently, Druid does not support pushing down predicates (condition and filter) past a Join (i.e. into 
Join's children). Druid only supports pushing predicates into the join if they originated from 
above the join. Hence, the location of predicates and filters in your Druid SQL is very important. 
Also, as a result of this, comma joins should be avoided.

#### Future work for joins

Joins are an area of active development in Druid. The following features are missing today but may appear in
future versions:

- Reordering of predicates and filters (pushing up and/or pushing down) to get the most performant plan.
- Preloaded dimension tables that are wider than lookups (i.e. supporting more than a single key and single value).
- RIGHT OUTER and FULL OUTER joins. Currently, they are partially implemented. Queries will run but results will not
always be correct.
- Performance-related optimizations as mentioned in the [previous section](#join-performance).
- Join algorithms other than broadcast hash-joins.
- Join condition on a column compared to a constant value.
- Join conditions on a column containing a multi-value dimension.