Are there non obvious differences between NVL and Coalesce in Oracle?
The obvious differences are that coalesce will return the first non null item in its parameter list whereas nvl only takes two parameters and returns the first if it is not null, otherwise it returns the second.
It seems that NVL may just be a 'Base Case" version of coalesce.
Am I missing something?
COALESCE
is more modern function that is a part of ANSI-92
standard.
NVL
is Oracle
specific, it was introduced in 80
's before there were any standards.
In case of two values, they are synonyms.
However, they are implemented differently.
NVL
always evaluates both arguments, while COALESCE
usually stops evaluation whenever it finds the first non-NULL
(there are some exceptions, such as sequence NEXTVAL
):
SELECT SUM(val)
FROM (
SELECT NVL(1, LENGTH(RAWTOHEX(SYS_GUID()))) AS val
FROM dual
CONNECT BY
level <= 10000
)
This runs for almost 0.5
seconds, since it generates SYS_GUID()
's, despite 1
being not a NULL
.
SELECT SUM(val)
FROM (
SELECT COALESCE(1, LENGTH(RAWTOHEX(SYS_GUID()))) AS val
FROM dual
CONNECT BY
level <= 10000
)
This understands that 1
is not a NULL
and does not evaluate the second argument.
SYS_GUID
's are not generated and the query is instant.