Table functions#

A table function is a function returning a table. It can be invoked inside the FROM clause of a query:

SELECT * FROM TABLE(my_function(1, 100))

The row type of the returned table can depend on the arguments passed with invocation of the function. If different row types can be returned, the function is a polymorphic table function.

Polymorphic table functions allow you to dynamically invoke custom logic from within the SQL query. They can be used for working with external systems as well as for enhancing Trino with capabilities going beyond the SQL standard.

For the list of built-in table functions available in Trino, see built in table functions.

Trino supports adding custom table functions. They are declared by connectors through implementing dedicated interfaces. For guidance on adding new table functions, see the developer guide.

Connectors offer support for different functions on a per-connector basis. For more information about supported table functions, refer to the connector documentation.

Built-in table functions#

exclude_columns(input => table, columns => descriptor) → table#

Excludes from table all columns listed in descriptor:

SELECT *
FROM TABLE(exclude_columns(
                        input => TABLE(orders),
                        columns => DESCRIPTOR(clerk, comment)))

The argument input is a table or a query. The argument columns is a descriptor without types.

Table function invocation#

You invoke a table function in the FROM clause of a query. Table function invocation syntax is similar to a scalar function call.

Function resolution#

Every table function is provided by a catalog, and it belongs to a schema in the catalog. You can qualify the function name with a schema name, or with catalog and schema names:

SELECT * FROM TABLE(schema_name.my_function(1, 100))
SELECT * FROM TABLE(catalog_name.schema_name.my_function(1, 100))

Otherwise, the standard Trino name resolution is applied. The connection between the function and the catalog must be identified, because the function is executed by the corresponding connector. If the function is not registered by the specified catalog, the query fails.

The table function name is resolved case-insensitive, analogically to scalar function and table resolution in Trino.

Arguments#

There are three types of arguments.

  1. Scalar arguments

They must be constant expressions, and they can be of any SQL type, which is compatible with the declared argument type:

factor => 42

  1. Descriptor arguments

Descriptors consist of fields with names and optional data types:

schema => DESCRIPTOR(id BIGINT, name VARCHAR)
columns => DESCRIPTOR(date, status, comment)

To pass null for a descriptor, use:

schema => CAST(null AS DESCRIPTOR)

  1. Table arguments

You can pass a table name, or a query. Use the keyword TABLE:

input => TABLE(orders)
data => TABLE(SELECT * FROM region, nation WHERE region.regionkey = nation.regionkey)

If the table argument is declared as set semantics, you can specify partitioning and ordering. Each partition is processed independently by the table function. If you do not specify partitioning, the argument is processed as a single partition. You can also specify PRUNE WHEN EMPTY or KEEP WHEN EMPTY. With PRUNE WHEN EMPTY you declare that you are not interested in the function result if the argument is empty. This information is used by the Trino engine to optimize the query. The KEEP WHEN EMPTY option indicates that the function should be executed even if the table argument is empty. Note that by specifying KEEP WHEN EMPTY or PRUNE WHEN EMPTY, you override the property set for the argument by the function author.

The following example shows how the table argument properties should be ordered:

input => TABLE(orders)
                    PARTITION BY orderstatus
                    KEEP WHEN EMPTY
                    ORDER BY orderdate

Argument passing conventions#

There are two conventions of passing arguments to a table function:

  • Arguments passed by name:

    SELECT * FROM TABLE(my_function(row_count => 100, column_count => 1))

In this convention, you can pass the arguments in arbitrary order. Arguments declared with default values can be skipped. Argument names are resolved case-sensitive, and with automatic uppercasing of unquoted names.

  • Arguments passed positionally:

    SELECT * FROM TABLE(my_function(1, 100))

In this convention, you must follow the order in which the arguments are declared. You can skip a suffix of the argument list, provided that all the skipped arguments are declared with default values.

You cannot mix the argument conventions in one invocation.

You can also use parameters in arguments:

PREPARE stmt FROM
SELECT * FROM TABLE(my_function(row_count => ? + 1, column_count => ?));

EXECUTE stmt USING 100, 1;