Druid connector#

The Druid connector allows querying an Apache Druid database from Trino.

Requirements#

To connect to Druid, you need:

  • Druid version 0.18.0 or higher.

  • Network access from the Trino coordinator and workers to your Druid broker. Port 8082 is the default port.

Configuration#

Create a catalog properties file that specifies the Druid connector by setting the connector.name to druid and configuring the connection-url with the JDBC string to connect to Druid.

For example, to access a database as example, create the file etc/catalog/example.properties. Replace BROKER:8082 with the correct host and port of your Druid broker.

connector.name=druid
connection-url=jdbc:avatica:remote:url=http://BROKER:8082/druid/v2/sql/avatica/

You can add authentication details to connect to a Druid deployment that is secured by basic authentication by updating the URL and adding credentials:

connection-url=jdbc:avatica:remote:url=http://BROKER:port/druid/v2/sql/avatica/;authentication=BASIC
connection-user=root
connection-password=secret

Now you can access your Druid database in Trino with the example catalog name from the properties file.

The connection-user and connection-password are typically required and determine the user credentials for the connection, often a service user. You can use secrets to avoid actual values in the catalog properties files.

Data source authentication#

The connector can provide credentials for the data source connection in multiple ways:

  • inline, in the connector configuration file

  • in a separate properties file

  • in a key store file

  • as extra credentials set when connecting to Trino

You can use secrets to avoid storing sensitive values in the catalog properties files.

The following table describes configuration properties for connection credentials:

Property name

Description

credential-provider.type

Type of the credential provider. Must be one of INLINE, FILE, or KEYSTORE; defaults to INLINE.

connection-user

Connection user name.

connection-password

Connection password.

user-credential-name

Name of the extra credentials property, whose value to use as the user name. See extraCredentials in Parameter reference.

password-credential-name

Name of the extra credentials property, whose value to use as the password.

connection-credential-file

Location of the properties file where credentials are present. It must contain the connection-user and connection-password properties.

keystore-file-path

The location of the Java Keystore file, from which to read credentials.

keystore-type

File format of the keystore file, for example JKS or PEM.

keystore-password

Password for the key store.

keystore-user-credential-name

Name of the key store entity to use as the user name.

keystore-user-credential-password

Password for the user name key store entity.

keystore-password-credential-name

Name of the key store entity to use as the password.

keystore-password-credential-password

Password for the password key store entity.

General configuration properties#

The following table describes general catalog configuration properties for the connector:

Property name

Description

Default value

case-insensitive-name-matching

Support case insensitive schema and table names.

false

case-insensitive-name-matching.cache-ttl

This value should be a duration.

1m

case-insensitive-name-matching.config-file

Path to a name mapping configuration file in JSON format that allows Trino to disambiguate between schemas and tables with similar names in different cases.

null

case-insensitive-name-matching.config-file.refresh-period

Frequency with which Trino checks the name matching configuration file for changes. This value should be a duration.

(refresh disabled)

metadata.cache-ttl

The duration for which metadata, including table and column statistics, is cached.

0s (caching disabled)

metadata.cache-missing

Cache the fact that metadata, including table and column statistics, is not available

false

metadata.cache-maximum-size

Maximum number of objects stored in the metadata cache

10000

write.batch-size

Maximum number of statements in a batched execution. Do not change this setting from the default. Non-default values may negatively impact performance.

1000

dynamic-filtering.enabled

Push down dynamic filters into JDBC queries

true

dynamic-filtering.wait-timeout

Maximum duration for which Trino will wait for dynamic filters to be collected from the build side of joins before starting a JDBC query. Using a large timeout can potentially result in more detailed dynamic filters. However, it can also increase latency for some queries.

20s

Appending query metadata#

The optional parameter query.comment-format allows you to configure a SQL comment that is sent to the datasource with each query. The format of this comment can contain any characters and the following metadata:

  • $QUERY_ID: The identifier of the query.

  • $USER: The name of the user who submits the query to Trino.

  • $SOURCE: The identifier of the client tool used to submit the query, for example trino-cli.

  • $TRACE_TOKEN: The trace token configured with the client tool.

The comment can provide more context about the query. This additional information is available in the logs of the datasource. To include environment variables from the Trino cluster with the comment , use the ${ENV:VARIABLE-NAME} syntax.

The following example sets a simple comment that identifies each query sent by Trino:

query.comment-format=Query sent by Trino.

With this configuration, a query such as SELECT * FROM example_table; is sent to the datasource with the comment appended:

SELECT * FROM example_table; /*Query sent by Trino.*/

The following example improves on the preceding example by using metadata:

query.comment-format=Query $QUERY_ID sent by user $USER from Trino.

If Jane sent the query with the query identifier 20230622_180528_00000_bkizg, the following comment string is sent to the datasource:

SELECT * FROM example_table; /*Query 20230622_180528_00000_bkizg sent by user Jane from Trino.*/

Note

Certain JDBC driver settings and logging configurations might cause the comment to be removed.

Domain compaction threshold#

Pushing down a large list of predicates to the data source can compromise performance. Trino compacts large predicates into a simpler range predicate by default to ensure a balance between performance and predicate pushdown. If necessary, the threshold for this compaction can be increased to improve performance when the data source is capable of taking advantage of large predicates. Increasing this threshold may improve pushdown of large dynamic filters. The domain-compaction-threshold catalog configuration property or the domain_compaction_threshold catalog session property can be used to adjust the default value of 32 for this threshold.

Procedures#

  • system.flush_metadata_cache()

    Flush JDBC metadata caches. For example, the following system call flushes the metadata caches for all schemas in the example catalog

    USE example.example_schema;
    CALL system.flush_metadata_cache();
    

Case insensitive matching#

When case-insensitive-name-matching is set to true, Trino is able to query non-lowercase schemas and tables by maintaining a mapping of the lowercase name to the actual name in the remote system. However, if two schemas and/or tables have names that differ only in case (such as “customers” and “Customers”) then Trino fails to query them due to ambiguity.

In these cases, use the case-insensitive-name-matching.config-file catalog configuration property to specify a configuration file that maps these remote schemas/tables to their respective Trino schemas/tables:

{
  "schemas": [
    {
      "remoteSchema": "CaseSensitiveName",
      "mapping": "case_insensitive_1"
    },
    {
      "remoteSchema": "cASEsENSITIVEnAME",
      "mapping": "case_insensitive_2"
    }],
  "tables": [
    {
      "remoteSchema": "CaseSensitiveName",
      "remoteTable": "tablex",
      "mapping": "table_1"
    },
    {
      "remoteSchema": "CaseSensitiveName",
      "remoteTable": "TABLEX",
      "mapping": "table_2"
    }]
}

Queries against one of the tables or schemes defined in the mapping attributes are run against the corresponding remote entity. For example, a query against tables in the case_insensitive_1 schema is forwarded to the CaseSensitiveName schema and a query against case_insensitive_2 is forwarded to the cASEsENSITIVEnAME schema.

At the table mapping level, a query on case_insensitive_1.table_1 as configured above is forwarded to CaseSensitiveName.tablex, and a query on case_insensitive_1.table_2 is forwarded to CaseSensitiveName.TABLEX.

By default, when a change is made to the mapping configuration file, Trino must be restarted to load the changes. Optionally, you can set the case-insensitive-name-mapping.refresh-period to have Trino refresh the properties without requiring a restart:

case-insensitive-name-mapping.refresh-period=30s

Type mapping#

Because Trino and Druid each support types that the other does not, this connector modifies some types when reading data.

Druid type to Trino type mapping#

The connector maps Druid types to the corresponding Trino types according to the following table:

Druid type to Trino type mapping#

Druid type

Trino type

Notes

STRING

VARCHAR

FLOAT

REAL

DOUBLE

DOUBLE

LONG

BIGINT

Except for the special _time column, which is mapped to TIMESTAMP.

TIMESTAMP

TIMESTAMP

Only applicable to the special _time column.

No other data types are supported.

Druid does not have a real NULL value for any data type. By default, Druid treats NULL as the default value for a data type. For example, LONG would be 0, DOUBLE would be 0.0, STRING would be an empty string '', and so forth.

Type mapping configuration properties#

The following properties can be used to configure how data types from the connected data source are mapped to Trino data types and how the metadata is cached in Trino.

Property name

Description

Default value

unsupported-type-handling

Configure how unsupported column data types are handled:

  • IGNORE, column is not accessible.

  • CONVERT_TO_VARCHAR, column is converted to unbounded VARCHAR.

The respective catalog session property is unsupported_type_handling.

IGNORE

jdbc-types-mapped-to-varchar

Allow forced mapping of comma separated lists of data types to convert to unbounded VARCHAR

SQL support#

The connector provides globally available and read operation statements to access data and metadata in the Druid database.

Table functions#

The connector provides specific table functions to access Druid.

query(varchar) -> table#

The query function allows you to query the underlying database directly. It requires syntax native to Druid, because the full query is pushed down and processed in Druid. This can be useful for accessing native features which are not available in Trino or for improving query performance in situations where running a query natively may be faster.

The native query passed to the underlying data source is required to return a table as a result set. Only the data source performs validation or security checks for these queries using its own configuration. Trino does not perform these tasks. Only use passthrough queries to read data.

As an example, query the example catalog and use STRING_TO_MV and MV_LENGTH from Druid SQL’s multi-value string functions to split and then count the number of comma-separated values in a column:

SELECT
  num_reports
FROM
  TABLE(
    example.system.query(
      query => 'SELECT
        MV_LENGTH(
          STRING_TO_MV(direct_reports, ",")
        ) AS num_reports
      FROM company.managers'
    )
  );

Note

The query engine does not preserve the order of the results of this function. If the passed query contains an ORDER BY clause, the function result may not be ordered as expected.