Using Functions and Operators

Description of user-defined and built-in functions and operators in LightDB-A Database.

• Using Functions in LightDB-A Database
• User-Defined Functions
• Built-in Functions and Operators
• Window Functions
• Advanced Aggregate Functions

Parent topic: Querying Data

Using Functions in LightDB-A Database

When you invoke a function in LightDB-A Database, function attributes control the execution of the function. The volatility attributes (IMMUTABLE, STABLE, VOLATILE) and the EXECUTE ON attributes control two different aspects of function execution. In general, volatility indicates when the function is run, and EXECUTE ON indicates where it is run. The volatility attributes are PostgreSQL based attributes, the EXECUTE ON attributes are LightDB-A Database attributes.

For example, a function defined with the IMMUTABLE attribute can be run at query planning time, while a function with the VOLATILE attribute must be run for every row in the query. A function with the EXECUTE ON MASTER attribute runs only on the coordinator instance, and a function with the EXECUTE ON ALL SEGMENTS attribute runs on all primary segment instances (not the coordinator).

These tables summarize what LightDB-A Database assumes about function execution based on the attribute.

You can display the function volatility and EXECUTE ON attribute information with the psql \df+ function command.

Refer to the PostgreSQL Function Volatility Categories documentation for additional information about the LightDB-A Database function volatility classifications.

For more information about EXECUTE ON attributes, see CREATE FUNCTION.

In LightDB-A Database, data is divided up across segments — each segment is a distinct PostgreSQL database. To prevent inconsistent or unexpected results, do not run functions classified as VOLATILE at the segment level if they contain SQL commands or modify the database in any way. For example, functions such as setval() are not allowed to run on distributed data in LightDB-A Database because they can cause inconsistent data between segment instances.

A function can run read-only queries on replicated tables (DISTRIBUTED REPLICATED) on the segments, but any SQL command that modifies data must run on the coordinator instance.

Note The hidden system columns (ctid, cmin, cmax, xmin, xmax, and gp_segment_id) cannot be referenced in user queries on replicated tables because they have no single, unambiguous value. LightDB-A Database returns a column does not exist error for the query.

To ensure data consistency, you can safely use VOLATILE and STABLE functions in statements that are evaluated on and run from the coordinator. For example, the following statements run on the coordinator (statements without a FROM clause):

SELECT setval('myseq', 201);
SELECT foo();

If a statement has a FROM clause containing a distributed table and the function in the FROM clause returns a set of rows, the statement can run on the segments:

SELECT * from foo();

LightDB-A Database does not support functions that return a table reference (rangeFuncs) or functions that use the refCursor data type.

Function Volatility and Plan Caching

There is relatively little difference between the STABLE and IMMUTABLE function volatility categories for simple interactive queries that are planned and immediately run. It does not matter much whether a function is run once during planning or once during query execution start up. But there is a big difference when you save the plan and reuse it later. If you mislabel a function IMMUTABLE, LightDB-A Database may prematurely fold it to a constant during planning, possibly reusing a stale value during subsequent execution of the plan. You may run into this hazard when using PREPAREd statements, or when using languages such as PL/pgSQL that cache plans.

User-Defined Functions

LightDB-A Database supports user-defined functions. See Extending SQL in the PostgreSQL documentation for more information.

Use the CREATE FUNCTION statement to register user-defined functions that are used as described in Using Functions in LightDB-A Database. By default, user-defined functions are declared as VOLATILE, so if your user-defined function is IMMUTABLE or STABLE, you must specify the correct volatility level when you register your function.

By default, user-defined functions are declared as EXECUTE ON ANY. A function that runs queries to access tables is supported only when the function runs on the coordinator instance, except that a function can run SELECT commands that access only replicated tables on the segment instances. A function that accesses hash-distributed or randomly distributed tables must be defined with the EXECUTE ON MASTER attribute. Otherwise, the function might return incorrect results when the function is used in a complicated query. Without the attribute, planner optimization might determine it would be beneficial to push the function invocation to segment instances.

When you create user-defined functions, avoid using fatal errors or destructive calls. LightDB-A Database may respond to such errors with a sudden shutdown or restart.

In LightDB-A Database, the shared library files for user-created functions must reside in the same library path location on every host in the LightDB-A Database array (coordinators, segments, and mirrors).

You can also create and run anonymous code blocks that are written in a LightDB-A Database procedural language such as PL/pgSQL. The anonymous blocks run as transient anonymous functions. For information about creating and running anonymous blocks, see the DO command.

Built-in Functions and Operators

The following table lists the categories of built-in functions and operators supported by PostgreSQL. All functions and operators are supported in LightDB-A Database as in PostgreSQL with the exception of STABLE and VOLATILE functions, which are subject to the restrictions noted in Using Functions in LightDB-A Database. See the Functions and Operators section of the PostgreSQL documentation for more information about these built-in functions and operators.

LightDB-A Database includes JSON processing functions that manipulate values the json data type. For information about JSON data, see Working with JSON Data.

Window Functions

The following built-in window functions are LightDB-A extensions to the PostgreSQL database. All window functions are immutable. For more information about window functions, see Window Expressions.

Advanced Aggregate Functions

The following built-in advanced aggregate functions are LightDB-A extensions of the PostgreSQL database. These functions are immutable.

Note The LightDB-A MADlib Extension for Analytics provides additional advanced functions to perform statistical analysis and machine learning with LightDB-A Database data. See LightDB-A MADlib Extension for Analytics in the LightDB-A Database Reference Guide.

SELECT departmzent_id, MEDIAN(salary) 
  FROM employees 
GROUP BY department_id; 

CREATE TABLE mymatrix (myvalue int[]);
INSERT INTO mymatrix 
   VALUES (array[[1,2],[3,4]]);
INSERT INTO mymatrix 
   VALUES (array[[0,1],[1,0]]);
SELECT sum(myvalue) FROM mymatrix;
 sum 
---------------
 {{1,3},{4,4}}