What is missing:

• persistent storage of functions (in the functions directory as a list of .sql files);
• DROP FUNCTION query;
• access rights for functions manipulation.

@Realist007 please add more tests for functionality.

1. Need to check how function will behave with constant arguments.
   For example CREATE FUNCTION custom_plus as (a, b) -> a + b;
   SELECT isConstant(custom_plus(1, 2)) should be true.
2. With non constant arguments.
3. Also need more tests to check for errors, it would be nice to add additional context for example SELECT custom_plus ('test', 'test').

• check for correct error messages when trying to create a function with the name of already existing builtin function;
• check for correct error messages when trying to drop existing builtin function;
• order of initialization on server startup if functions are dependent on each other;
• check for correct error messages when trying to create recursive function;
• the case for distributed queries when cluster nodes contain different implementation of the same function;
• make sure that the function is JIT compiled.

#11

Internal documentation ticket: DOCSUP-13375

will you support recursive function?

CREATE FUNCTION f3 AS a -> multiIf(a < 2, 1, f3(a) + f3(a + 1))

Query id: 38b7b875-5877-4081-ac79-ad0493cc6923

0 rows in set. Elapsed: 0.001 sec. 

Received exception from server (version 21.11.1):
Code: 611. DB::Exception: Received from localhost:9000. DB::Exception: You cannot create recursive function. (CANNOT_CREATE_RECURSIVE_FUNCTION)

@l1t1 Not in near time.

User defined SQL functions are substituted (as expression) firstly and only then optionally compiled and evaluated.
We cannot do infinite substitution, that's why recursion is not supported.

Any ideas of how to support this?

CREATE FUNCTION UDFTestArrayMap AS (d) -> arrayMap( x -> JSONExtractKeysAndValues(x, 'String'), d )

DB::Exception: Identifier `x` does not exist in arguments. (UNKNOWN_IDENTIFIER) (version 21.10.2.15 (official build))

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