Introduction to SQL routines#

Примечание

Ниже приведена оригинальная документация Trino. Скоро мы ее переведем на русский язык и дополним полезными примерами.

A SQL routine is a custom, user-defined function authored by a user of Trino and written in the SQL routine language. You can declare the routine body within a FUNCTION block as inline routines or catalog routines.

Inline routines#

An inline routine declares and uses the routine within a query processing context. The routine is declared in a WITH block before the query:

WITH
  FUNCTION abc(x integer)
    RETURNS integer
    RETURN x * 2
SELECT abc(21);

Inline routine names must follow SQL identifier naming conventions, and cannot contain . characters.

The routine declaration is only valid within the context of the query. A separate later invocation of the routine is not possible. If this is desired, use a catalog routine.

Multiple inline routine declarations are comma-separated, and can include routines calling each other, as long as a called routine is declared before the first invocation.

WITH
  FUNCTION abc(x integer)
    RETURNS integer
    RETURN x * 2,
  FUNCTION xyz(x integer)
    RETURNS integer
    RETURN abc(x) + 1
SELECT xyz(21);

Note that inline routines can mask and override the meaning of a built-in function:

WITH
  FUNCTION abs(x integer)
    RETURNS integer
    RETURN x * 2
SELECT abs(-10); -- -20, not 10!

Catalog routines#

You can store a routine in the context of a catalog, if the connector used in the catalog supports routine storage. In this scenario, the following commands can be used:

Catalog routines must use a name that combines the catalog name and schema name with the routine name, such as example.default.power for the power routine in the default schema of the example catalog.

Invocation must use the fully qualified name, such as example.default.power.

SQL environment configuration#

Configuration of the sql.default-function-catalog and sql.default-function-schema SQL environment properties allows you to set the default storage for SQL routines. The catalog and schema must be added to the sql.path as well. This enables users to call SQL routines and perform all SQL routine management without specifying the full path to the routine.

Примечание

Use the Memory коннектор in a catalog for simple storing and testing of your SQL routines.

Routine declaration#

Refer to the documentation for the FUNCTION keyword for more details about declaring the routine overall. The routine body is composed with statements from the following list:

Statements can also use built-in functions and operators as well as other routines, although recursion is not supported for routines.

Find simple examples in each statement documentation, and refer to the example documentation for more complex use cases that combine multiple statements.

Примечание

User-defined functions can alternatively be written in Java and deployed as a plugin. Details are available in the developer guide.

Labels#

Routines can contain labels as markers for a specific block in the declaration before the following keywords:

  • CASE

  • IF

  • LOOP

  • REPEAT

  • WHILE

The label is used to name the block in order to continue processing with the ITERATE statement or exit the block with the LEAVE statement. This flow control is supported for nested blocks, allowing to continue or exit an outer block, not just the innermost block. For example, the following snippet uses the label top to name the complete block from REPEAT to END REPEAT:

top: REPEAT
  SET a = a + 1;
  IF a <= 3 THEN
    ITERATE top;
  END IF;
  SET b = b + 1;
  UNTIL a >= 10
END REPEAT;

Labels can be used with the ITERATE and LEAVE statements to continue processing the block or leave the block. This flow control is also supported for nested blocks and labels.

Recommendations#

Processing routines can potentially be resource intensive on the cluster in terms of memory and processing. Take the following considerations into account when writing and running SQL routines:

  • Some checks for the runtime behavior of routines are in place. For example, routines that use take longer to process than a hardcoded threshold are automatically terminated.

  • Avoid creation of arrays in a looping construct. Each iteration creates a separate new array with all items and copies the data for each modification, leaving the prior array in memory for automated clean up later. Use a lambda expression instead of the loop.

  • Avoid concatenating strings in a looping construct. Each iteration creates a separate new string and copying the old string for each modification, leaving the prior string in memory for automated clean up later. Use a lambda expression instead of the loop.

  • Most routines should declare the RETURNS NULL ON NULL INPUT characteristics unless the code has some special handling for null values. You must declare this explicitly since CALLED ON NULL INPUT is the default characteristic.

Limitations#

The following limitations apply to SQL routines.

  • Routines must be declared before than can be referenced.

  • Recursion cannot be declared or processed.

  • Mutual recursion can not be declared or processed.

  • Queries cannot be processed in a routine.