Postgres externally supplied parameters
@a_horse_with_no_name is right in the comments, from the docs


  SPI_execute_with_args executes a command that might include references to externally supplied parameters. The command text refers to a parameter as $n, and the call specifies data types and values for each such symbol. read_only and count have the same interpretation as in SPI_execute.


So you can do 

SELECT * FROM partition_master WHERE foo = 5;


or using prepared statements,

SELECT * FROM partition_master WHERE foo = $1;


But you can't do...

SELECT * FROM whatever AS w
JOIN partition_master AS p
WHERE p.foo = w.bar;