Decode Vs Case: context switching?

Question

So I was told recently that among other reasons, CASE is "better" than Decode in SQL statements because Decode context switches to PL/SQL to perform the checks.
I can't find anything in the documentation to support this.
this site here:
http://www.dba-oracle.com/oracle_news/2005_11_23_case_decode_machinations.htm
mentions that one of the disadvantages of decode is that it's post-retrieval, but it also seems to mention that so is CASE.
anyone have any idea where someone may have got the "context switching" idea from?

Answer 1

have often wondered why you would use CASE in PL/SQL when it has IF THEN control structures. Yes, you could, however readability would suffer. But what is more important CASE has a form where expression is evaluated only once:
SQL> SET SERVEROUTPUT ON
SQL> DECLARE
  2      X NUMBER;
  3  BEGIN
  4      PKG1.CNT := 2;
  5      X := CASE PKG1.F1
  6             WHEN 1 THEN 1
  7             WHEN 2 THEN 2
  8             WHEN 3 THEN 3
  9           END;
10      DBMS_OUTPUT.PUT_LINE('X = ' || X);
11      DBMS_OUTPUT.PUT_LINE('PKG1.CNT = ' || PKG1.CNT);
12  END;
13  /
Call to PKG1.F1
X = 3
PKG1.CNT = 3
PL/SQL procedure successfully completed.
SQL> DECLARE
  2      X NUMBER;
  3  BEGIN
  4      PKG1.CNT := 2;
  5      IF PKG1.F1 = 1
  6        THEN X := 1;
  7      ELSIF PKG1.F1 = 2
  8        THEN X := 2;
  9      ELSIF PKG1.F1 = 3
10        THEN X := 3;
11      END IF;
12      DBMS_OUTPUT.PUT_LINE('X = ' || X);
13      DBMS_OUTPUT.PUT_LINE('PKG1.CNT = ' || PKG1.CNT);
14  END;
15  /
Call to PKG1.F1
Call to PKG1.F1
Call to PKG1.F1
X =
PKG1.CNT = 5
PL/SQL procedure successfully completed.
SQL> In such case you would have to introduce a temp variable:
SQL> CREATE OR REPLACE
  2  PACKAGE PKG1
  3  IS
  4  CNT NUMBER;
  5  FUNCTION F1 RETURN NUMBER;
  6  END;
  7  /
Package created.
SQL> CREATE OR REPLACE
  2  PACKAGE BODY PKG1
  3  IS
  4  FUNCTION F1 RETURN NUMBER
  5  IS
  6  BEGIN
  7  DBMS_OUTPUT.PUT_LINE('Call to PKG1.F1');
  8  CNT := CNT + 1;
  9  RETURN CNT;
10  END;
11  END;
12  /
Package body created.
SQL> SET SERVEROUTPUT ON
SQL> DECLARE
  2      X NUMBER;
  3  BEGIN
  4      PKG1.CNT := 2;
  5      X := CASE PKG1.F1
  6             WHEN 1 THEN 1
  7             WHEN 2 THEN 2
  8             WHEN 3 THEN 3
  9           END;
10      DBMS_OUTPUT.PUT_LINE('X = ' || X);
11      DBMS_OUTPUT.PUT_LINE('PKG1.CNT = ' || PKG1.CNT);
12  END;
13  /
Call to PKG1.F1
X = 3
PKG1.CNT = 3
PL/SQL procedure successfully completed.
SQL> DECLARE
  2      X NUMBER;
  3      TMP NUMBER;
  4  BEGIN
  5      PKG1.CNT := 2;
  6      TMP := PKG1.F1;
  7      IF TMP = 1
  8        THEN X := 1;
  9      ELSIF TMP = 2
10        THEN X := 2;
11      ELSIF TMP = 3
12        THEN X := 3;
13      END IF;
14      DBMS_OUTPUT.PUT_LINE('X = ' || X);
15      DBMS_OUTPUT.PUT_LINE('PKG1.CNT = ' || PKG1.CNT);
16  END;
17  /
Call to PKG1.F1
X = 3
PKG1.CNT = 3
PL/SQL procedure successfully completed.
SQL> SY.