Presto

Supported since 5.0.2.

Through the Presto (opens new window) connector, Ontop is able to construct VKGs on Presto databases.

Limitations & Exceptions

WARNING

Presto does not provide information about integrity constraints. Make sure to provide this information in order to avoid very inefficient queries. We recommend using lenses for this purpose.

• Accessing struct fields with the "dot operator" is not supported (see below).

Database Connection

The following shows the content of a sample .properties file that can be used to connect Ontop to Presto:

jdbc.url = jdbc:presto://localhost:8080/defaultdatabase
jdbc.user = user
jdbc.password = password
jdbc.driver = com.facebook.presto.jdbc.PrestoDriver

In case Presto is set up without authentication, the jdbc.user and jdbc.password lines can also be left out.

Nested Type Support

Ontop implements explicit compatibility with the Presto array type array(t). When used with the flatten lens, it is able to automatically infer the type of the result column.

The flatten lens cannot be used on arrays stored as JSON-encoded columns in the database. For such use cases, the column first has to be converted to an array type.

Struct Access

In Presto, individual ROW objects can be accessed by SQL expressions using the "dot operator" on the ROW column. In Ontop, this feature is not currently supported.

Should any of their elements still be required, then a workaround can be performed by first transforming the struct into a JSON object and then accessing it using JSON functions.

Example:

my_row.my_attribute JSON_EXTRACT(CAST(my_struct AS JSON), '$.my_attribute')

Elements of MAP objects can be accessed using the [] operator, as in

SELECT my_map['my_attribute'] FROM ...