Обсуждение: How to quote the COALESCE function?

Roman Scherer <roman@burningswell.com> writes:
> Can someone explain to me what's the difference between quoting
> the `upper` and the `coalesce` function?

COALESCE is a keyword.

> What I found so far is, that the `upper` function can be found in
> the `pg_proc` table but not `coalesce`.

Yup.

> Does this mean that `coalesce` isn't a classical function and I
> shouldn't quote it? Is it instead a keyword, as described in
> the "Lexical Structure" section of the docs [2]? How can I find
> out which other functions are not meant to be quoted?

Yes, yes, and you already found one good way: if it doesn't have
a pg_proc entry then it's a special case of some sort or other.

Have you considered only quoting the function name if it actually
needs it, ie, contains special characters?

            regards, tom lane

Roman Scherer <roman@burningswell.com> writes:

> Hello,
>
> I'm building a DSL in Clojure for SQL and specifically PostgreSQL
> [1]. When building a SQL statement that contains a function call
> I always quote the function name with \" in case the function
> name contains any special characters. Here's an example:
>
>   (select db ['(upper "x")])
>   ;=> ["SELECT \"upper\"(?)" "x"]
>
> This worked fine so far, but today I found a case that doesn't
> work as expected, the COALESCE function.
>
>   (select db ['(coalesce nil 0)])
>   ;=> ["SELECT \"coalesce\"(NULL, 0)"]
>
> Can someone explain to me what's the difference between quoting
> the `upper` and the `coalesce` function? I can execute the
> following statements via psql, and it works as expected:
>
>   SELECT upper ('x');
>   SELECT "upper"('x');
>   SELECT coalesce(NULL, 1);
>
> But as soon as I try this with `coalesce` I get an error:
>
>   SELECT "coalesce"(NULL, 1);


While not a precise answer to your question, it may be of interest to
note that coalesce is *not* a function.

It is a language construct with a function-like syntax.

select distinct proname from pg_proc where proname in ('coalesce', 'lower');
 proname
---------
 lower
(1 row)

>
>   ERROR:  function coalesce(unknown, integer) does not exist
>   LINE 1: SELECT "coalesce"(NULL, 1);
>                  ^
>   HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
>
> What I found so far is, that the `upper` function can be found in
> the `pg_proc` table but not `coalesce`.
>
>   SELECT proname FROM pg_proc WHERE proname ILIKE 'upper';
>   SELECT proname FROM pg_proc WHERE proname ILIKE 'coalesce';
>
> Does this mean that `coalesce` isn't a classical function and I
> shouldn't quote it? Is it instead a keyword, as described in
> the "Lexical Structure" section of the docs [2]? How can I find
> out which other functions are not meant to be quoted?
>
> I'm aware that I do not need to quote the `coalesce` and `upper`
> functions and I may change my strategy for quoting functions names.
>
> Thanks for you help, Roman.
>
> [1] https://github.com/r0man/sqlingvo
> [2] http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html
>

--
Jerry Sievers
Postgres DBA/Development Consulting
e: postgres.consulting@comcast.net
p: 312.241.7800

On Tue, Mar 29, 2016 at 12:38 PM, Pavel Stehule <pavel.stehule@gmail.com> wrote:

> The coalesce is one few functions implemented by special rule in
> PostgreSQL parser.

In the SQL standard the COALESCE feature is not listed as a
function; it is listed as one of the short forms of CASE
expression.  While it has function-like syntax, thinking of it as a
function is semantically incorrect.

  COALESCE(a, b)

is supposed to be semantically equivalent to:

  CASE WHEN a is not null THEN a ELSE b END

Among other things, that means that this statement should not
generate a divide by zero error:

  SELECT COALESCE(1, 1/0);

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company