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Re: [HACKERS] dollar quoting

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>,"Patches (PostgreSQL)" <pgsql-patches(at)postgresql(dot)org>
Subject: Re: [HACKERS] dollar quoting
Date: 2004-02-14 20:04:31
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackerspgsql-patches

Tom Lane wrote:

>Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>>I ended up not using a regex, which seemed to be a little heavy handed, 
>>but just writing a small custom recognition function, that should (and I 
>>think does) mimic the pattern recognition for these tokens used by the 
>>backend lexer.
>I looked at this and realized that it still doesn't do very well at
>distinguishing $foo$ from other random uses of $.  The problem is that
>looking back at just the immediately preceding character isn't enough
>context to tell whether a $ is part of an identifier.  Consider the
>	a42$foo$
>This is a legal identifier according to PG 7.4.  But how about
>	42$foo$
>This is a syntax error in 7.4, and we propose to redefine it as an
>integer literal '42' followed by a dollar-quote start symbol.

The test in the patch I sent is this:

            else if (!dol_quote && valid_dolquote(line+i) &&
                     (i == 0 ||
                      ! ((line[i-prevlen] & 0x80) != 0 ||
                         isalnum(line[i-prevlen]) ||
                         line[i-prevlen] == '_' ||
                         line[i-prevlen] == '$' )))

The test should not succeed anywhere in the string '42$foo$'.

Note that psql does not change any '$foo$' at all - it just passes it to 
the backend. The reason we need this at all in psql is that it has to 
detect the end of a statement, and it has to prompt correctly, and to do 
that it needs to know if we are in a quote (single, double, dollar) or a 

psql does not detect many syntax errors, or even lexical errors - that 
is the job of the backend - rightly so, I believe.

>There's no way to tell these apart with a single-character lookback,
>or indeed any fixed number of characters of lookback.

I'm still not convinced, although maybe there's something I'm not getting.

>I begin to think that we'll really have to bite the bullet and convert
>psql's input parser to use flex.  If we're not scanning with exactly the
>same rules as the backend uses, we're going to get the wrong answers.

Interacting with lexer states would probably be ... unpleasant. Matching 
a stream oriented lexer with a line oriented CLI would be messy I suspect.



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