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CREATE FUNCTION (TVF)

Creates a user-defined table-valued function (TVF). A TVF is a callable routine that accepts input parameters, executes a single SELECT statement in the function body, and returns a single table-type value.

Syntax

[USING database_name] CREATE [OR REPLACE] FUNCTION function_name ( [parameter_list] )
  RETURNS TABLE AS RETURN function_body ;

parameter_list:
  variable_name data_type [data_type_modifier] [, ...]

data_type_modifier:
  DEFAULT default_value | NOT NULL | NULL | COLLATE collation_name

Arguments

OR REPLACE

If specified, replaces a TVF if one already exists with the same name.

function_name

The name of the function. Function names must be unique within a database, and cannot be duplicates of names for other TVFs, tables, views, user-defined functions (UDFs), user-defined aggregate functions (UDAFs), or stored procedures.

Table Valued Function names are case-sensitive. This matches the behavior of table names.

parameter_list

Input parameters are optional. Any number of input parameters can be specified, and each must be delimited by a comma (,). Each input parameter must be given a unique identifier name within the scope of the function.

The following example shows how to declare a single input parameter:

CREATE FUNCTION single_param_example(a INT) ...

The following example demonstrates how to declare more than one input parameter:

CREATE FUNCTION multi_param_example(a INT, b VARCHAR(255), c DECIMAL(19,4)) ...

Only scalar data types are allowed for TVF input parameters, and each valid type is enumerated in the syntax block above. Non-scalar types, such as ARRAY or RECORD, cannot be declared as input parameters for TVFs.

Default values can be specified for input parameters by using the DEFAULT constraint. Consider the following example:

CREATE FUNCTION default_example(a INT DEFAULT 5) ...

Input parameters can also specify data type modifiers, namely NOT NULL, NULL, and COLLATE. Consider the following example:

CREATE FUNCTION modifier_example(a TEXT NOT NULL COLLATE utf8_bin) ...

data_type

Any scalar-valued data type as described in the Data Types topic. Non-scalar valued types are not allowed in TVFs, including ARRAY and RECORD types.

function_body

The function body of a TVF may contain any valid SELECT statement.

Remarks

Table-valued functions (TVFs) in MemSQL provide a mechanism to return a result set from a single user-defined SELECT statement, much like a view. Unlike views, however, TVFs accept scalar-type input parameters that can be used inside the function body. Consider the following example TVF, which accepts a DECIMAL input type and returns the items in a store’s inventory that match the desired price:

CREATE FUNCTION inventory_with_price(price DECIMAL(19,4))
  RETURNS TABLE AS RETURN
    SELECT * FROM store_inventory WHERE item_price = price;

This TVF specifies an input parameter named price, which is used to query the store_inventory table and find all items of the specified price. The function is executed in the following way:

SELECT * FROM inventory_with_price(1.9900);

Once the inventory_with_price function has been created, it can be executed with different DECIMAL input values to query the store_inventory table in a dynamic, reusable manner.

TVFs can also reference other TVFs in the function body. When a referenced TVF is executed inside the function body of a TVF, the result set from the referenced TVF can be queried like a table. Consider the following example where tvf_2 references tvf_1:

CREATE FUNCTION tvf_1(a INT)
  RETURNS TABLE AS RETURN
    SELECT * FROM store_inventory
    LIMIT a;

CREATE FUNCTION tvf_2()
  RETURNS TABLE AS RETURN
    SELECT * FROM tvf_1(10)
    GROUP BY item_price
    ORDER BY item_price
    LIMIT 10;

In the example above, a table named store_inventory contains a item_price column. The tvf_1 function is referenced in tvf_2 and its logical result set is queried as a table type.

Security and Permissions

TVFs in MemSQL use the Definer Security Model. The TVF’s definer must have the permissions to execute the SELECT statement that makes up the TVF’s function body.

Limitations

TVFs are limited in the following ways:

SELECT Statement Limitations

TVFs may only contain a single SELECT statement; any other DML or DDL statement in the TVF function body is invalid. Multiple SELECT statements are currently not supported.

Unique Column Names

Each column in a TVF’s logical result set must have a unique name. Duplicate column names are invalid, and will result in an error.

Scalar Input Parameters

TVF input parameters may only be scalar value types, such as INT, VARCHAR, and so on. UDF-specific data types such as ARRAY or RECORD cannot be used as input parameter types.

Self-Calling (Recursive Calls)

While a TVF can reference other TVFs in the function body, a TVF may not call itself, directly or indirectly.

Database Scope

Unlike UDFs, TVFs may only be executed within the scope of the database in which they are created. Cross-database TVF execution is not supported. Additionally, referenced TVFs in a TVF function body cannot execute TVFs in another database. For example, the following TVF is invalid:

CREATE FUNCTION cross_database_invalid()
  RETURNS TABLE AS RETURN
    SELECT * FROM otherdb.other_tvf();

Related Topics

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