From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
cc: pgsql-hackers@postgresql.org
Subject: Re: A really subtle lexer bug
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Comments: In-reply-to Andrew Gierth <andrew@tao11.riddles.org.uk>
	message dated "Mon, 20 Aug 2018 17:58:49 +0100"
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Date: Thu, 23 Aug 2018 10:55:15 -0400
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Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "Andrew" == Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
>  Andrew> I guess the fix is to extend the existing special case code
>  Andrew> that checks for one character left after removing trailing [+-]
>  Andrew> and also check for the two-character ops "<>" ">=" "<=" "=>"
>  Andrew> "!=".

> Patch attached.
> This fixes two bugs: first the mis-lexing of two-char ops as mentioned
> originally; second, the O(N^3) lexing time of strings of - or +
> characters is reduced to O(N^2) (in practice it's better than O(N^2)
> once N gets large because the bison stack gets blown out, ending the
> loop early).

Looks reasonable offhand (didn't test).  A couple of thoughts:

* Some regression tests exercising these code paths might be a good thing.

* There should likely be a comment near where EQUALS_GREATER and
friends are defined, pointing out that if we add any more multi-character
operators with special precedences, this code has to be taught about them.

			regards, tom lane