Hi,
(I didn't read all the updates, sorry if this was already mentioned)
NVL
<https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/NVL.html#GUID-3AB61E54-9201-4D6A-B48A-79F4C4A034B2>
evaluates all arguments but COALESCE
<https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/COALESCE.html#GUID-3F9007A7-C0CA-4707-9CBA-1DBF2CDE0C87>
(returns
the first non-null argument) stops at the first non-null argument.
I learned this during UKOUG Techfest 19 where I attended a "SQL Syntax:
Oracle vs. ANSI. Fight!" by Alex Nuijten & Chris Saxon.
Useful conference. ROI++ :D
---
Maris Elsins
On Mon, Dec 9, 2019 at 9:17 AM Andy Sayer <andysayer@xxxxxxxxx> wrote:
“In hindsight, I guess I'm more flabbergasted by the fact that I have
either completely forgotten about this, or have never run into this before.
And both of those worry me equally :)”
I’m in the “didn’t everyone know this difference?” camp. I think the main
problem here is that the issue is such a simple difference, it’s one you’ll
either learn very early on or never (or when you hit something that causes
a problem). The description of coalesce doing short circuiting is in the
docs but coalesce is such an early learnt function that you probably don’t
look it up in the docs.
There’s probably a whole bunch of these nuances with simple functions that
you either learn very early on in your career or don’t.
In fairness to NVL, I think it’s mostly used to replace nulls with a
constant, so short circuit evaluation wouldn’t be a big deal most of the
time (although probably never hurts).
Just my 2c,
Andy
On Mon, 9 Dec 2019 at 03:18, Stefan Knecht <knecht.stefan@xxxxxxxxx>
wrote:
Yeah that's exactly it, Chris.
I can see it making sense to evaluate both expressions in a SQL context,
for perhaps some or other performance benefit.
But if you look at it purely from a procedural perspective:
x := nvl(some_var, some_func);
It doesn't make a whole lot of sense to execute f2 if f1 is not null.
In this particular case, I used this simply as a shorthand for if
some_var is null then some_func else some_var end if;
And I can definitely now answer the question Michael asked in his post
"how difficult would it be to find that bug?": hella, hella difficult.
As for Mark's comments - the reason we do this is the fact that the code
needs to run in two distinct environments. In one, the value we test will
never be null, and in some it will always be null. In the latter case, we
need to use the function to do a little bit of extra work. Using coalesce()
in this case in lieu of nvl() will do exactly what we need.
In hindsight, I guess I'm more flabbergasted by the fact that I have
either completely forgotten about this, or have never run into this before.
And both of those worry me equally :)
Cheers
Stefan
On Mon, Dec 9, 2019 at 1:37 AM Chris Taylor <
christopherdtaylor1994@xxxxxxxxx> wrote:
oooooh like I said, I figured I was missing something. LOL
I didn't realize the issue was more about the original NVL/NVL2
functions.
I can definitely see that side of it on the one hand, but on the other,
those are also designed that way and have been around for a long time.
I guess I'm just surprised anyone would be grumpy about a provided
function , when that function documented behavior is , erm, well,
documented :)
Chris
On Sun, Dec 8, 2019, 11:12 AM Michael D O'Shea/Woodward Informatics Ltd <
woodwardinformatics@xxxxxxxxxxxxxxxx> wrote:
Hi Chris, not wishing to put words into Stefan’s mouth, I believe he is
just pointing out an unexpected behavioural difference between two inbuilt
Oracle functions.
The surprise is that NVL, NVL2, ... DECODE would be DESIGNED IN THE
FIRST PLACE to execute an expression or function twice. It just makes no
sense. I cannot think of another programming language where a form of a
null coalescing operator would execute a function/expression twice. This is
far from normal behaviour or expectation. Actually it is a man-trap,
especially if the function caused side effects from dual execution or order
of execution.
Stefan asked why.
I postulate just sloppy programming by Oracle.
Of course chronologically NVL etc. came first. Perhaps the programmers
that wrote COALESCE did it properly second time around, or perhaps it is
prescribed this way in the ANSI/ISO specs?
Mike
Michael D. O’Shea, https://www.linkedin.com/in/michaeldoshea
Woodward Informatics Ltd, http://www.strychnine.co.uk
Am 08.12.2019 um 16:52 schrieb Chris Taylor <
christopherdtaylor1994@xxxxxxxxx>:
I'm a little confused by this thread (though I understand the effect) .
Coalesce by design/creation is supposed to stop at the first not null
evaluation. That's what it's for. It's not for evaluating all given
arguments passed to it.
So if you have code that needs to evaluate "all the things", then
coalesce isn't the code you're looking for, correct?
I know I'm probably missing something or not tracking, but that
function is designed to stop evaluating at the first not null evaluation.
Chris
On Sun, Dec 8, 2019, 10:01 AM Michael D O'Shea/Woodward Informatics Ltd
<woodwardinformatics@xxxxxxxxxxxxxxxx> wrote:
So it appears that both NVL() and NVL2() are executing the functionin the second argument, regardless of whether the first argument is NULL
or
not.
Yes.
I ranted about this a few years ago now here:
http://www.strychnine.co.uk/2016/10/junk-dna-junk-oracle-part-0000002/
Does anyone see a reason why this is?
My conclusion …. just sloppy programming. Of course given the codebase
built upon NVL, other NULL handling functions, and those with a failover
value (*eg*. DECODE), the behaviour cannot be fixed without breaking
the existing user codebase, as you have observed.
Mike
Michael D. O’Shea, https://www.linkedin.com/in/michaeldoshea
Woodward Informatics Ltd, http://www.strychnine.co.uk
Am 08.12.2019 um 15:37 schrieb Mark W. Farnham <mwf@xxxxxxxx>:
As for repairing your code, if a bit of code is only valid for an
inbound null argument, that check for is not null turning the guts into a
no-op is the cohesive solution (also known in ancient times as a
"firewall."). You can do that with a wrapper and protect the fragile
function within a package body or use a wide variety of ways to project
the
fragility, or you can do it in the function. Try to avoid doing it twice
(or n) though: repetitive firewalls instead of encapsulation can become
significant and might underflow routine detection in profilers despite
being wasteful in aggregate. Avoid also replicating the "if is not null"
inline source code wrapper, which is an example of writing redundant
"sprinkled all over" application code that retains the overhead even after
the function is encapsulated.
THE PRECISE formulation of where to put "firewalls" has in the past
been something of a religious war. I am agnostic on that religious war
other than holding it should be done a single way in a given application
suite.
mwf
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [
mailto:oracle-l-bounce@xxxxxxxxxxxxx ;<oracle-l-bounce@xxxxxxxxxxxxx>]
On Behalf Of Stefan Koehler
Sent: Sunday, December 08, 2019 3:26 AM
To: knecht.stefan@xxxxxxxxx
Cc: oracle-l-freelists
Subject: Re: Interesting difference between nvl() and coalesce()
Hello Stefan,
ah OK. Just to get the complete picture, please be also aware about
the impact on the CBO if you gonna change the code.
https://jonathanlewis.wordpress.com/2018/02/13/coalesce-v-nvl/
Best Regards
Stefan Koehler
Independent Oracle performance consultant and researcher
Website: http://www.soocs.de
Twitter: @OracleSK
Stefan Knecht <knecht.stefan@xxxxxxxxx> hat am 8. Dezember 2019 um
09:20 geschrieben:
Perhaps to add a bit of detail - here it wasn't just a performance
thing. It literally broke the code, because we have logic in the function
that's called in the second argument of nvl(), and that logic is only
valid
if the first argument that came in, was indeed NULL.
Sigh.
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