Why a function within a SELECT FROM DUAL is faster?
612338Oct 6 2010 — edited Oct 8 20101)I have a SELECT with a function. Ex:
"SELECT DISTINCT function(...) FROM table"
2)I changed the SELECT and put the function inside a SUB-SELECT with FROM DUAL. Ex:
"SELECT DISTINCT (SELECT function(...) FROM DUAL) FROM table"
3)The second SELECT is faster than the first.
I discovered this problem on my tables and my functions. So I did a generic example for this forum with a common function and large table. My results:
Table has 2.196.058 records and the field is a VARCHAR2:
SELECT DISTINCT SUBSTR(field, 2) FROM Table -> Executed in 110 seconds
SELECT DISTINCT (SELECT SUBSTR(field, 2) FROM DUAL) FROM Table -> Executed in 39 seconds
Why "SELECT DISTINCT (SELECT function(...) FROM DUAL) FROM table" is faster than "SELECT DISTINCT function(...) FROM table"
thanks,
Fernando