Oracle Differences between NVL and Coalesce
NVL and COALESCE are used to achieve the same functionality of providing a default value in case the column returns a NULL.
The differences are:
- NVL accepts only 2 arguments whereas COALESCE can take multiple arguments
- NVL evaluates both the arguments and COALESCE stops at first occurrence of a non-Null value.
- NVL does a implicit datatype conversion based on the first argument given to it. COALESCE expects all arguments to be of same datatype.
- COALESCE gives issues in queries which use UNION clauses. Example below
- COALESCE is ANSI standard where as NVL is Oracle specific.
Examples for the third case. Other cases are simple.
select nvl('abc',10) from dual;
would work as NVL will do an implicit conversion of numeric 10 to string.
select coalesce('abc',10) from dual;
will fail with Error - inconsistent datatypes: expected CHAR got NUMBER
Example for UNION use-case
SELECT COALESCE(a, sysdate)
from (select null as a from dual
union
select null as a from dual
);
fails with ORA-00932: inconsistent datatypes: expected CHAR got DATE
SELECT NVL(a, sysdate)
from (select null as a from dual
union
select null as a from dual
) ;
succeeds.
More information : http://www.plsqlinformation.com/2016/04/difference-between-nvl-and-coalesce-in-oracle.html
There is also difference is in plan handling.
Oracle is able form an optimized plan with concatenation of branch filters when search contains comparison of nvl
result with an indexed column.
create table tt(a, b) as
select level, mod(level,10)
from dual
connect by level<=1e4;
alter table tt add constraint ix_tt_a primary key(a);
create index ix_tt_b on tt(b);
explain plan for
select * from tt
where a=nvl(:1,a)
and b=:2;
explain plan for
select * from tt
where a=coalesce(:1,a)
and b=:2;
nvl:
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 52 | 2 (0)| 00:00:01 |
| 1 | CONCATENATION | | | | | |
|* 2 | FILTER | | | | | |
|* 3 | TABLE ACCESS BY INDEX ROWID| TT | 1 | 26 | 1 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IX_TT_B | 7 | | 1 (0)| 00:00:01 |
|* 5 | FILTER | | | | | |
|* 6 | TABLE ACCESS BY INDEX ROWID| TT | 1 | 26 | 1 (0)| 00:00:01 |
|* 7 | INDEX UNIQUE SCAN | IX_TT_A | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(:1 IS NULL)
3 - filter("A" IS NOT NULL)
4 - access("B"=TO_NUMBER(:2))
5 - filter(:1 IS NOT NULL)
6 - filter("B"=TO_NUMBER(:2))
7 - access("A"=:1)
coalesce:
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 26 | 1 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| TT | 1 | 26 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IX_TT_B | 40 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("A"=COALESCE(:1,"A"))
2 - access("B"=TO_NUMBER(:2))
Credits go to http://www.xt-r.com/2012/03/nvl-coalesce-concatenation.html.
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.
NVL will do an implicit conversion to the datatype of the first parameter, so the following does not error
select nvl('a',sysdate) from dual;
COALESCE expects consistent datatypes.
select coalesce('a',sysdate) from dual;
will throw a 'inconsistent datatype error'