​
Overview

The pg_get_expr() is a system catalog information function that retrieves the internal form of an individual expression, such as the default value for a column.

​
Syntax

There are two available syntax versions of the pg_get_expr() function: 
SELECT pg_get_expr('expr_text', relation_oid);
Both versions of the pg_get_expr() function return an empty string "".

​
Parameters

The following parameters are required to execute this function:
  • expr_text: expression for which you want to obtain the internal representation (can be any string value)
  • relation_oid: OID (Object Identifier) of the table the expression belongs to (integer type)
  • pretty_bool: boolean value determining whether to format the expression in a more human-readable format (TRUE) or not (FALSE)

​
Example

For the needs of this section, first we will create a sample table named employees 
CREATE TABLE employees (
    id INT,
    name TEXT,
    salary TEXT
);
Then we will get the OID of the table 
SELECT oid FROM pg_class WHERE relname = 'employees';

 oid  
------
 1018
As the last step, we will retrieve the internal form for the salary column using pg_get_expr() function 
-- Version 1
SELECT pg_get_expr('salary', 1018);

-- Version 2
SELECT pg_get_expr('salary', 1018, TRUE);
By executing any of the queries above, we will get the following output: 
 pg_get_expr 
-------------