"Andrew Hammond" <andrew.george.hammond at gmail.com> writes:
> On 8/30/07, Jan Wieck <JanWieck at yahoo.com> wrote:
>           On 8/29/2007 2:52 PM, Dmitry Koterov wrote:
>      > And much worse - seems such DDL with comments breaks the whole
>      > replication (fortunately I have run it on a test server, not in production).
>      > I watched sl_event table on a subscriber and noticed that the whole DDL
>      > (with comments included) are passed and saved in the subscriber.
>      
>      Where in the ANSI SQL standard does it allow -- comments nested inside
>      of /* ... */ comments?
>
>
> I can see having problems with "real" nested comments.
> /*
>   layer one
>   /*
>     layer two
>   */
>   I'll bet this doesn't work
> */
> Or, if you want to get really stupid, like this.
> -- this is a stupid place to /* start
> a multi-line coment
> */

Note that PostgreSQL supports nested comments, which, I should note, C
*does not.*

That is, in C, the following comment is *broken*:

   /* Here is a comment /* that nests */ */

That comment ends, in C, after the first "*/".  In effect, in C, the
following is a single comment:

   /* Here is a comment /* that nests */ c++;

So the first comment that you describe is, as far as C, is invalid,
but PostgreSQL does accept it.

Your second example:
  -- quote to end of a line that includes /* what looks like the start of a C-style comment
   multilining
   */

is treated, by PostgreSQL, as invalid.  The "--" hides everything
until the end of the line, and that includes the "/*", so that its
parser does not recognize there being any beginning to what you
imagined was going to begin commenting line 2.

> But honestly, a parser that fails because of comment characters inside a comment... that's kinda broken.
> /* this may be stupid
> -- but, really it should still work. */

PostgreSQL recognizes this as a valid comment.  Since the parser is
already in "comment mode" when it gets to "--", that does NOT hide the
rest of the line.

>           Just because Postgres itself allows it doesn't mean that every piece of
>      software working with Postgres must be aware of this, does it?
>
> I think that for any software which works _only_ with Postgres, it
> is reasonable to assume that it will attempt to be as close to the
> Postgres interpretation / implementation of SQL as possible. One of
> the ways to test DDL before applying it via slonik is to run it with
> a rollback on each node. If a script passes this test, then it seems
> to me that it's reasonable to expect it to work via EXECUTE DDL.
> Hmm, maybe that's an idea for a new feature "EXECUTE DDL DRYRUN" or
> something of that nature...

The nature of this particular problem actually makes my head hurt.

- My first thought was, "no big deal, I'll just get the parser to
  recognize C-style comments."  I have code not yet checked in that
  does this, and which survives something of a "torture test."

Here's a pretty reasonable torture test:

select * from
/************ *foooo*   /- *
/   here is a tremendously ugly C-style comment
* * * * * /
-- **{***(***[****/
b;

However, in looking at things deeper, then comes the "ow!" part.

C doesn't support nested comments, but *PostgreSQL does*!!!

Nested /* /* */ */ comments are somewhat harder to support, and when
this actually *IS NOT* supporting a "C style comment" (as there's
demonstrably different behaviour), it frankly strikes me as silly to
head deeply into this.

There is a conspicuous bug in that the parser presently never comes
out of "C-style comment mode;" I'm happy enough to address that.

But I am also loathe to go to extremes to support functionality that
is also deviant from what people's expectations will generally be.
-- 
let name="cbbrowne" and tld="linuxfinances.info" in name ^ "@" ^ tld;;
http://cbbrowne.com/info/slony.html
"There are  three kinds  of program statements:  sequence, repetition,
and seduction."