substring odd behavior

On Thu, Jan 27, 2022 at 7:22 PM Regina Obe <lr@pcorp.us> wrote:

Is this intentional behavior?

-- I can do this
SELECT substring('3.2.0' from '[0-9]*\.([0-9]*)\.');

-- But can't do this gives error syntax error at or near "from"
SELECT pg_catalog.substring('3.2.0' from '[0-9]*\.([0-9]*)\.');

select pg_catalog.trim(leading 'hi' from 'hi david'); -- syntax error at or
near "leading" (returns ' david' if pg_catalog is omitted). I also tested
position(text in text) and get a syntax failure at the second text argument.

Generalizing from three examples, it seems the SQL Standard defined
functions that use keywords cannot be reliably called with a schema prefix.

This seems likely to be intended (or at least not worth avoiding) behavior
given the special constraints these functions place on the parser. But I
expect someone more authoritative than I on the subject to chime in. For
me, adding another sentence to Chapter 9.4 (I don't know if other
categories of functions have this dynamic) after we say "SQL defines some
string functions that use key words...PostgreSQL also provides versions of
these functions that use the regular function invocation syntax." The
schema qualification would seem to be part of "regular function invocation
syntax" only.

I'd consider adding "Note that the ability to specify the pg_catalog schema
in front of the function name only applies to the regular function
invocation syntax." Though it isn't like our users are tripping over this
either. Maybe noting it in the Syntax chapter would be more appropriate.
I'd rather not leave the status quo behavior without documenting it
somewhere (but all the better if it can be fixed).

It probably isn't worth adding a section to the Syntax chapter for
"Irregular Function Calls (SQL Standard Mandated)" but I'd entertain the
thought.

David J.

"David G. Johnston" <david.g.johnston@gmail.com> writes:

On Thu, Jan 27, 2022 at 7:22 PM Regina Obe <lr@pcorp.us> wrote:

Is this intentional behavior?
-- I can do this
SELECT substring('3.2.0' from '[0-9]*\.([0-9]*)\.');
-- But can't do this gives error syntax error at or near "from"
SELECT pg_catalog.substring('3.2.0' from '[0-9]*\.([0-9]*)\.');

Generalizing from three examples, it seems the SQL Standard defined
functions that use keywords cannot be reliably called with a schema prefix.

The syntax with keywords instead of commas is defined in the SQL standard,
and it is defined there without any schema qualification. They seem to
think that "substring" is also a keyword of sorts, and that's how we
implement it.

In short: you can call substring() with the SQL syntax, which is a
special-purpose production that does not involve any schema name,
or you can call it as an ordinary function with ordinary function
notation. You can't mix pieces of those notations.

regards, tom lane

On Jan 27, 2022, at 7:06 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

In short: you can call substring() with the SQL syntax, which is a
special-purpose production that does not involve any schema name,
or you can call it as an ordinary function with ordinary function
notation. You can't mix pieces of those notations.

Beware that your choice of grammar interacts with search_path considerations. If you use what looks like a regular function call, the search_path will be consulted, but if you use the "from" based syntax, the one from pg_catalog will be used:

SET search_path = substr_test, pg_catalog;
SELECT substring('first', 'second');
substring
----------------------------
substr_test: first, second
(1 row)

SELECT substring('first' FROM 'second');
substring
-----------

(1 row)

SET search_path = pg_catalog, substr_test;
SELECT substring('first', 'second');
substring
-----------

(1 row)

SELECT substring('first' FROM 'second');
substring
-----------

(1 row)

SELECT substr_test.substring('first', 'second');
substring
----------------------------
substr_test: first, second
(1 row)

SELECT substr_test.substring('first' FROM 'second');
ERROR: syntax error at or near "FROM"
LINE 1: SELECT substr_test.substring('first' FROM 'second');

—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Hi,

On Thu, Jan 27, 2022 at 07:54:49PM -0700, David G. Johnston wrote:

On Thu, Jan 27, 2022 at 7:22 PM Regina Obe <lr@pcorp.us> wrote:

Is this intentional behavior?

-- I can do this
SELECT substring('3.2.0' from '[0-9]*\.([0-9]*)\.');

-- But can't do this gives error syntax error at or near "from"
SELECT pg_catalog.substring('3.2.0' from '[0-9]*\.([0-9]*)\.');

select pg_catalog.trim(leading 'hi' from 'hi david'); -- syntax error at or
near "leading" (returns ' david' if pg_catalog is omitted). I also tested
position(text in text) and get a syntax failure at the second text argument.

Generalizing from three examples, it seems the SQL Standard defined
functions that use keywords cannot be reliably called with a schema prefix.

Yes, I don't have a copy of the standard but I think that they define such
constructs as part of the language and not plain function calls, so you can't
schema qualify it.

That's how it's internally implemented, and the SUBSTRING( FOR / FROM / ESCAPE
) is a syntactic sugar over pg_catalog.substring().