What happens when a function is called over and over again with the same arguments? It is executed over and over again, which is quite easy to understand. PostgreSQL allows a function to declare a level of immutability so that the query executor can make some assumptions to improve the speed of the execution.
To better understand this concept, consider the f_human_file_size() function shown in Listing 46. The function accepts a numeric argument and produces a text representation of that argument:
testdb=> SELECT f_human_file_size( 1234567890 ); f_human_file_size------------------- 1.15MB
Reading the function implementation, it is clear that such a function will provide the same result for the same input every time and it would ...
