Oracle: difference between deterministic and result_cache

From the translator: I decided to start my journey to Habr not with an attempt to write some unique text from scratch, but with the translation of a relatively fresh (from 08/17/2020) article by the classic PL / SQL development Stephen Feuerstein, in which he discusses the difference in sufficient detail between the two main variants of the PL / SQL function result cache. I hope this translation will be helpful for many developers getting started with Oracle technologies.

Introduction

Sooner or later, any seasoned Oracle developer is asked a question like:

I don't understand what exactly is the difference between deterministic and result_cache. Do they have different use cases? I use deterministic in many functions that get data from lookup tables. Do I need to use the result_cache keyword instead of deterministic?

I thought it would be worth writing about the differences between these two possibilities. First of all, let's make sure we all have the same understanding of when a function is deterministic.

The Wikipedia provides the following definition of a deterministic algorithm:

A deterministic algorithm is an algorithm that returns the same set of outputs for the same set of inputs, while performing the same sequence of actions.

In other words, a deterministic subroutine (procedure or function) has no side effects. By passing a specific set of values ​​as input parameters, you will always get the same result at the output, regardless of when, where, or how often you call this subroutine.

A reasonable question is, what is a side effect of a PL / SQL function? At a minimum (the list is not exhaustive):

• any (that is, any) DML operator

  
  
  
  
  

• Using a variable declared outside of this function (that is, global, out-of-scope aka "global")

  
  
  
  
  

• call any non-deterministic subroutine

  
  
  
  
  

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DECLARE
   n   NUMBER := 0;
BEGIN
   FOR rec IN (SELECT pass_number (100)
                 FROM all_objects
                WHERE ROWNUM < 6)
   LOOP
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   END LOOP;

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END;
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BEGIN
   DBMS_OUTPUT.PUT_LINE ('Returned ' || pass_number (100));
   DBMS_OUTPUT.PUT_LINE ('Returned ' || pass_number (200));
   DBMS_OUTPUT.PUT_LINE ('Returned ' || pass_number (300));
   DBMS_OUTPUT.PUT_LINE ('Returned ' || pass_number (100));
   DBMS_OUTPUT.PUT_LINE ('Returned ' || pass_number (200));
   DBMS_OUTPUT.PUT_LINE ('Returned ' || pass_number (300));
END;
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