Mastering PostgreSQL 9.6

Reducing the number of function calls

In many cases, performance is bad because functions are called way too often. I cannot stress this point too much: calling things too often is the main reason for bad performance. When you create a function, you can choose from three types of functions: volatile, stable, and immutable. Here is an example:

test=# SELECT random(), random();       random       |      random        -------------------+-------------------  0.276252629235387 | 0.710661871358752 (1 row) test=# SELECT now(), now();               now              |              now               -------------------------------+-------------------------------  2016-12-16 12:57:17.135751+01 | 2016-12-16 12:57:17.135751+01 (1 row) test=# SELECT pi();         pi         ------------------  3.14159265358979 (1 row) 

A volatile function ...