User Defined Transform

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User Defined Transform

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A user-defined transform is a user-defined function that returns a set of rows or a stream of rows. Its input arguments can be scalars or cursors. A cursor is an object representing a subquery, which the user-defined transform can use to read the subquery results. The subquery can be relational or streaming.

The following table describes the differences between user-defined functions, procedures, and transforms:

Type of UDR

Created by ...

Accepts ...

Returns ...

Comments

User-Defined Function (UDF)

CREATE FUNCTION

 

Accepts ... Zero or more parameters

 

Returns ... Scalar value

--- A UDF is used in a scalar expression; it takes (0 .. n) scalar arguments and returns a scalar value.

--- When used in a query, a scalar expression is evaluated for each row.

--- SQLstream allows the creation of Java or SQL functions.

User-Defined Procedure (UDP)

CREATE PROCEDURE

 

Accepts ... Zero or more parameters

 

Returns ... No values returned

A UDP is just a UDF that returns no value; it is evaluated for a side effect.

User-Defined Transform (UDX)

CREATE FUNCTION

 

Accepts ... Zero or more Relations

 

Returns ... A Streaming Relation

--- A UDX is different: its output is a stream of rows, and its inputs can be scalars or be streams. In SQL,

--- a streaming argument to a UDX is represented as

     CURSOR(SELECT STREAM ...)

--- and a scalar is represented by any scalar expression.

The invocation of a UDX is quite often preceded by the keyword TABLE. For example:

 SELECT STREAM *

  FROM TABLE(

   filterSignal( CURSOR( SELECT STREAM * FROM "RawSignals")));