Oracle grammar ambiguities and conflicts

"matt blackmon" <mattblackmon@hotmail.com>
9 Jan 2006 23:54:39 -0500

          From comp.compilers

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From: "matt blackmon" <mattblackmon@hotmail.com>
Newsgroups: comp.compilers
Date: 9 Jan 2006 23:54:39 -0500
Organization: Compilers Central
References: 01-07-118
Keywords: parse, question



>I am attempting to write a parser for Oracle 7.3.4 SQL/PLSQL using
>Visual Parse++ 4.0. I have run into a problem where non-reserved
>keywords must be included as identifiers, causing millions of conflicts
>in the grammar. Does anyone have experience with either using VP++
>or with writing grammars for SQL?
>
>Thanks,
>
>John Fairhall
>Quest Software.
>[That's a famous problem. There's a variety of less than fabulous
>solutions, hacks in the lexer to guess when to return a keyword and
>when to return a symbol, parser backup if it gags on a keyword, or
>bushy parsers that put all of the allowed keywords into the grammar
>in places where they can be used as symbols. -John]


Sorry for resurrecting an old post from 2001, but I am wrestling with
this same famous problem and have had no luck (beyond the original
post) finding help on this famous problem. I'm currently working on
an oracle 10g sql parser using flex/bison. Of the three solutions
mentioned by the moderator (are there more?), here are the stumbling
blocks I've encountered:




1) "parser backup if it gags on a keyword" -- not feasible in
bison/flex (to my knowledge), and most tools capable of producing backups
use exception based processing, which results in unacceptable performance
for my task.


2) "hacks in the lexer" -- trying this currently, but I find the regex
I use matching too much, or not catching enough. Additionally, trying lexer
"states" seems to replicate all the work in the parser for determining the
context for determining whether the word is a keyword or symbol. Should I
have the parser switch the state of the lexer depending on which rule it is
pursuing? This seems messy, causing some instances where I'd have to back
up the lexer and re-lex.


3) "bushy parsers that put all of the allowed keywords into the
grammar in places where they can be used as symbols" -- I'm currently doing
this in my parser (I think), but it leads to some ambiguities in some cases,
particularly with words like DEFAULT.


Lastly, would a LL grammar be more befitting this problem than a LR?


Any feedback or pointers you could give me would be greatly appreciated as
my searching of this group, other groups, and the internet as a whole has
been fruitless.


Thanks,
Matt Blackmon
mattblackmon AT NOhotmailJUNK.com


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