5. PL/pgSQL Trigger Functions | PostgreSQL Server Programming - Second Edition

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PostgreSQL Server Programming - Second Edition - Second Edition

By : Dar, Krosing, Jim Mlodgenski

4.5 (8)

PostgreSQL Server Programming - Second Edition

4.5 (8)

By: Dar, Krosing, Jim Mlodgenski

Overview of this book

This book is for moderate to advanced PostgreSQL database professionals who wish to extend PostgreSQL, utilizing the most updated features of PostgreSQL 9.4. For a better understanding of this book, familiarity with writing SQL, a basic idea of query tuning, and some coding experience in your preferred language is expected.

Preface

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Free Chapter

1. What Is a PostgreSQL Server?

1. What Is a PostgreSQL Server?

Why program in the server?

About this book's code examples

Moving beyond simple functions

Managing related data with triggers

Auditing changes

Data cleaning

Custom sort orders

Programming best practices

Caching

Wrapping up – why program in the server?

Summary

2. Server Programming Environments

2. Server Programming Environments

Cost of acquisition

Availability of developers

Licensing

Predictability

Community

Procedural languages

Summary

3. Your First PL/pgSQL Function

3. Your First PL/pgSQL Function

Why PL/pgSQL?

The structure of a PL/pgSQL function

Conditional expressions

Returning a record

Acting on the function's results

Summary

4. Returning Structured Data

4. Returning Structured Data

Sets and arrays

Returning sets

Using a set returning function

Functions based on views

OUT parameters and records

A summary of the RETURN SETOF variants

Returning cursors

Other ways to work with structured data

Summary

5. PL/pgSQL Trigger Functions

5. PL/pgSQL Trigger Functions

Creating the trigger function

Working on a simple "Hey, I'm called" trigger

The audit trigger

Disallowing DELETE

Disallowing TRUNCATE

Modifying the NEW record

The immutable fields trigger

Controlling when a trigger is called

Visibility

Variables passed to the PL/pgSQL TRIGGER function

Summary

6. PostgreSQL Event Triggers

6. PostgreSQL Event Triggers

Use cases for creating event triggers

Creating event triggers

Creating an audit trail

Preventing schema changes

A roadmap of event triggers

Summary

7. Debugging PL/pgSQL

7. Debugging PL/pgSQL

Manual debugging with RAISE NOTICE

Visual debugging

Summary

8. Using Unrestricted Languages

8. Using Unrestricted Languages

Are untrusted languages inferior to trusted ones?

Can you use untrusted languages for important functions?

Will untrusted languages corrupt the database?

Why untrusted?

Quick introduction to PL/Python

Thinking out of the "SQL database server" box

Summary

9. Writing Advanced Functions in C

9. Writing Advanced Functions in C

The simplest C function – return (a + b)

Adding functionality to add(int, int)

Basic guidelines for writing C code

Error reporting from C functions

Running queries and calling PostgreSQL functions

Handling records as arguments or returned values

Fast capturing of database changes

Doing something at commit/rollback

Synchronizing between backends

Writing functions in C++

Additional resources for C

Summary

10. Scaling Your Database with PL/Proxy

10. Scaling Your Database with PL/Proxy

Creating a simple single-server chat

Dealing with success – splitting tables over multiple databases

Connection Pooling

Summary

11. PL/Perl – Perl Procedural Language

11. PL/Perl – Perl Procedural Language

When to use PL/Perl

Installing PL/Perl

A simple PL/Perl function

Passing and returning non-scalar types

Writing PL/Perl triggers

Untrusted Perl

Summary

12. PL/Tcl – Tcl Procedural Language

12. PL/Tcl – Tcl Procedural Language

Installing PL/Tcl

A simple PL/Tcl function

Passing and returning arrays

Passing composite-type arguments

Accessing databases

Writing PL/Tcl triggers

Untrusted Tcl

Summary

13. Publishing Your Code as PostgreSQL Extensions

13. Publishing Your Code as PostgreSQL Extensions

When to create an extension

Unpackaged extensions

Extension versions

The .control file

Building an extension

Installing an extension

Viewing extensions

Publishing your extension

Installing an extension from PGXN

Summary

14. PostgreSQL as an Extensible RDBMS

14. PostgreSQL as an Extensible RDBMS

What can't be extended?

Creating a new operator

Creating index access methods

Creating user-defined aggregates

Using foreign data wrappers

Summary

Index

Index

Disallowing DELETE

What if our business requirements are such that the data can only be added and modified in some tables, but not deleted?

One way to handle this will be to just revoke the DELETE rights on these tables from all the users (remember to also revoke DELETE from PUBLIC), but this can also be achieved using triggers because of reasons such as auditing or returning custom exception messages.

A generic cancel trigger can be written as follows:

CREATE OR REPLACE FUNCTION cancel_op() 
  RETURNS TRIGGER AS $$ 
BEGIN 
    IF TG_WHEN = 'AFTER' THEN 
        RAISE EXCEPTION 'YOU ARE NOT ALLOWED TO % ROWS IN %.%', 
          TG_OP, TG_TABLE_SCHEMA, TG_TABLE_NAMENAME; 
    END IF; 
    RAISE NOTICE '% ON ROWS IN %.% WON'T HAPPEN', 
          TG_OP, TG_TABLE_SCHEMA, TG_TABLE_NAMENAME; 
    RETURN NULL; 
END; 
$$ LANGUAGE plpgsql;

The same trigger function can be used for both the BEFORE and AFTER triggers. If you use it as a BEFORE trigger, the operation is skipped with a message. However...

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