Re: BUG #1329: Bug in IF-ELSEIF-ELSE construct

Tom Lane wrote:
> There is no ELSEIF construct.

Sure, but it would be nice to throw a syntax error rather than silently
accepting the function. Unfortunately the way PL/PgSQL's parser works
doesn't make this very easy. (BTW, I think that fixing how we do parsing
would be one of the prime motivations for rewriting PL/PgSQL. One
possibility would be to integrate the PL/PgSQL parser into the main SQL
parser, although there may be a cleaner way to improve PL/PgSQL parsing.)

In any case, given this function:

create or replace function foo() returns int as
'
#option dump
begin
if 5 > 5 then
return 10;
elseif 5 > 6 then
return 15;
else
return 20;
end if;
end;' language 'plpgsql';

We produce this parsetree: (helpfully dumped via the undocumented
"#option dump" feature)

Functions statements:
2:BLOCK <<*unnamed*>>
3: IF 'SELECT 5 > 5' THEN
4: RETURN 'SELECT 10'
5: EXECSQL 'elseif 5 > 6 then 15 15'
ELSE
8: RETURN 'SELECT 20'
ENDIF
END -- *unnamed*

One way to fix the specific bug reported here would be to add K_ELSEIF
to the PL/PgSQL lexer, and then throw a syntax error in the stmt_else
production. But that is a very limited fix: if the user specifies any
other word in the place of 'elseif', we will not throw a syntax error.

Another solution would be to teach the PL/PgSQL lexer to recognize the
initial tokens of every SQL statement (SELECT, UPDATE, and so forth).
Right now we just assume an unrecognized word must be the beginning of a
SQL statement; if we taught the lexer about the initial tokens of all
legal SQL statements, we could reject unrecognized words. But this is
kind of ugly as well, as it means duplicating the knowledge about what
constitutes a legal SQL statement in multiple places.

Alternatively, we could arrange to have the PL/PgSQL parser pass a block
of text it has identified as a possible SQL statement to the main SQL
parser; if it produces a syntax error, we can pass that syntax error
back to the user. I'm not sure if this would have any negative
ramifications, though.

Comments?

(BTW, another thing this example exposes is that we don't issue warnings
about trivially-dead-code, such as statements in a basic block that
follow a RETURN. This would probably be also worth doing.)

-Neil