Understanding Postgres Event Triggers and Schema Creation

Introduction to Postgres Event Triggers

Postgres event triggers are a powerful feature that allows developers to respond to specific events occurring within their database schema. These triggers can be used for a wide range of purposes, from auditing changes to enforcing data consistency. In this article, we will explore the basics of Postgres event triggers and how they relate to schema creation.

The CREATE FUNCTION Statement

To create an event trigger in Postgres, you must first define a function using the CREATE FUNCTION statement. This function should take the necessary parameters and return the expected data type.

The following example demonstrates a basic CREATE FUNCTION statement:

CREATE FUNCTION trigger_after_create_schema()
RETURNS event_trigger LANGUAGE plpgsql
AS $$
BEGIN
  GRANT USAGE ON SCHEMA TG_TABLE_SCHEMA TO "api_server";
  GRANT SELECT, INSERT, UPDATE, DELETE ON ALL TABLES IN SCHEMA TG_TABLE_SCHEMA TO "api_server";
END;
$$;

In this example, the function is named trigger_after_create_schema and returns an event trigger. The LANGUAGE plpgsql clause specifies that the function should be written in PostgreSQL’s procedural language.

Defining the Event Trigger

To define the event trigger, we use the CREATE EVENT TRIGGER statement. This statement specifies the event to which the trigger will respond and the function that should be executed when the event occurs.

The following example demonstrates a basic CREATE EVENT TRIGGER statement:

CREATE EVENT TRIGGER after_create_schema
ON ddl_command_end
WHEN TAG IN ('CREATE SCHEMA')
EXECUTE PROCEDURE trigger_after_create_schema();

In this example, the trigger is named after_create_schema and responds to the ddl_command_end event. The WHEN TAG IN ('CREATE SCHEMA') clause specifies that the trigger should only be executed when a CREATE SCHEMA command is issued.

Understanding the Error Message

When we attempt to create this function and event trigger, we receive an error message indicating that no schema has been selected to create in. This error message can be confusing, as it suggests that there is an issue with the schema creation process itself.

However, upon closer inspection, we realize that the error message is actually a red herring. The problem lies not with the schema creation process, but rather with the fact that the CREATE SCHEMA command is being executed within a function that returns an event trigger.

Dynamic SQL and Function Execution

To resolve this issue, we must use dynamic SQL to execute the necessary commands within the function. This allows us to dynamically grant privileges on the newly created schema.

The following example demonstrates how to modify the CREATE FUNCTION statement to use dynamic SQL:

create or replace 
function trigger_after_create_schema()
 returns event_trigger 
language plpgsql
as $$
declare 
   k_grant_usage constant text = 
     'grant usage on schema % to "api_server"'; 
   k_grant_dml   constant text = 
     'grant select, insert, update, delete on all tables in schema tg_table_schema to "api_server"'; 
      
   l_ddl_stmt text;
begin
   l_ddl_stmt := format(k_grant_usage,tg_table_schema);
   raise notice 'Running statement: %',l_ddl_stmt; 
   execute l_ddl_stmt; 
  
   l_ddl_stmt := format(k_grant_dml,tg_table_schema);
   raise notice 'Running statement: %',l_ddl_stmt; 
   execute  l_ddl_stmt; 
end;
$$;

In this example, we use the format function to dynamically create SQL statements that grant privileges on the newly created schema. We then use the execute function to run these statements.

Conclusion

Postgres event triggers are a powerful feature that allows developers to respond to specific events occurring within their database schema. By understanding how to define and execute functions that return event triggers, we can create custom solutions for a wide range of use cases.

In this article, we explored the basics of Postgres event triggers and how they relate to schema creation. We demonstrated how to modify the CREATE FUNCTION statement to use dynamic SQL and function execution, allowing us to resolve the error message and successfully grant privileges on newly created schemas.

By following these steps and using the techniques outlined in this article, you can create custom event triggers that meet your specific needs and ensure the integrity of your database schema.

Last modified on 2024-03-14