How to See Variable Data in PostgreSQL Functions

1. RAISE NOTICE: This approach sends a message to the client (like the psql terminal) indicating the variable's value. You can use it within PL/pgSQL functions, a procedural language extension for PostgreSQL. Here's an example:

CREATE OR REPLACE FUNCTION print_var(var_name text) RETURNS void AS $$
BEGIN
  RAISE NOTICE 'Value: %', var_name;
END;
$$ LANGUAGE plpgsql;

SELECT print_var('This is my variable');

Running this function will display:

NOTICE:  Value: This is my variable

1. SELECT with subquery: This method involves a subquery that doesn't return any results but uses RAISE NOTICE within it. Here's an example:

SELECT 1 AS dummy;

WITH temp_data AS (
  SELECT RAISE NOTICE 'Variable value: %', my_variable;
)

SELECT * FROM temp_data; 

This will also display a notice similar to the first approach.

CREATE OR REPLACE FUNCTION print_var(var_name text) RETURNS void AS $$
BEGIN
  RAISE NOTICE 'Value: %', var_name; -- Print the variable value
END;
$$ LANGUAGE plpgsql;

SELECT print_var('This is my variable'); -- Call the function

Using RAISE NOTICE within a subquery (SELECT with subquery):

SELECT 1 AS dummy; -- Dummy select to avoid syntax errors

WITH temp_data AS (
  SELECT RAISE NOTICE 'Variable value: %', my_variable; -- Print inside subquery
)

SELECT * FROM temp_data; -- Doesn't return anything, just raises the notice

Explanation:

• In both examples, the RAISE NOTICE statement is used. It sends a non-fatal message to the client (psql terminal) indicating the variable's value.
• The first example defines a function print_var that takes a text variable var_name as input. Inside the function, it uses RAISE NOTICE to print the value of var_name. The function itself doesn't return anything (RETURNS void). Finally, the function is called with the string "This is my variable".
• The second example uses a Common Table Expression (CTE) named temp_data. Inside the CTE definition, there's a SELECT statement with RAISE NOTICE printing the value of the variable my_variable. However, this SELECT doesn't return any data (*) as it's primarily for raising the notice. The final SELECT * FROM temp_data acts as a trigger to execute the CTE and raise the notice.

Important Note:

• Replace my_variable in both examples with the actual variable you want to print the value of.
• These methods are for debugging purposes within PL/pgSQL functions. The message won't be part of the actual query results.

1. pg_log: This function allows you to write messages to the server log file. It's useful for more persistent logging compared to notices which are temporary. Here's an example:

CREATE OR REPLACE FUNCTION log_var(var_name text) RETURNS void AS $$
BEGIN
  pg_log('info', 'Variable value: %', var_name);
END;
$$ LANGUAGE plpgsql;

SELECT log_var('This is my variable');

This will write a message similar to "Variable value: This is my variable" to the server log.

1. pg_notify/LISTEN: This method involves sending notifications on a channel and listening for them in the client application. It allows for more complex interaction but requires additional setup. Here's a basic example:

CREATE OR REPLACE FUNCTION notify_var(var_name text) RETURNS void AS $$
BEGIN
  SELECT pg_notify('my_channel', 'Variable value: || quote_literal(var_name)');
END;
$$ LANGUAGE plpgsql;

SELECT notify_var('This is my variable');

-- In psql client (assuming you've listened to the channel)
LISTEN my_channel;

This will send a notification with the variable value to the channel my_channel. You'll need to set up LISTEN on the client-side (psql) to receive the notification.

1. Using RETURN: If your function needs to return the variable value, you can simply use the RETURN statement. This is the most straightforward method for functions that need to output data. Here's an example:

CREATE OR REPLACE FUNCTION get_var() RETURNS text AS $$
BEGIN
  RETURN 'This is my variable';
END;
$$ LANGUAGE plpgsql;

SELECT get_var();

This function will return the string "This is my variable" as the result.