Transform Functions (UDTFs)

A User-Defined Transform Function (UDTF) lets you transform a table of data into another table. It reads one or more arguments (treated as a row of data), and returns zero or more rows of data consisting of one or more columns. A UDTF can produce any number of rows as output. However, each row it outputs must be complete. Advancing to the next row without having added a value for each column produces incorrect results.

The schema of the output table does not need to correspond to the schema of the input table—they can be totally different. The UDTF can return any number of output rows for each row of input.

UDTFs can only be used in the SELECT list that contains just the UDTF call and a required OVER clause. A multi-phase UDTF can make use of partition columns (PARTITION BY), but other UDTFs cannot.

UDTFs are run after GROUP BY, but before the final ORDER BY, when used in conjunction with GROUP BY and ORDER BY in a statement. The ORDER BY clause may contain only columns or expressions that are in a window partition clause (see Window Partitioning).

UDTFs can take up to 1600 parameters (input columns). Attempting to pass more parameters to a UDTF results in an error.

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