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Doing psql's lexing with flex

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-patches(at)postgreSQL(dot)org
Subject: Doing psql's lexing with flex
Date: 2004-02-17 22:43:40
Message-ID: 18389.1077057820@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-patches
I got interested enough in the psql-with-flex problem to go off and
solve it.  Attached is a working patch, which I'm now debating whether
to apply.  Comments solicited...

The patch removes about 200 lines of very spaghetti-ish code in
mainloop.c.  However, it adds an 875-line flex source file, which
might be thought a bad tradeoff :-(.  One bright spot is that about
half of that total is a direct copy of the main backend lexer, so
it's not really as much new, separately maintainable code as all that.
Also, Andrew Dunstan's patch for supporting dollar-quoting would add
about 100 lines to mainloop.c, versus only a dozen or so lines in the
flex implementation.  Once that's taken into account I don't think there
is a lot of difference in effective SLOC to maintain.  I'm also of the
opinion that the new C code in psqlscan.l is much more straightforward
than the code removed from mainloop.c, though having just written it,
I'm no doubt pretty biased.

Bruce was asking about speed.  On normal-size queries I cannot measure
any difference at all.  For testing purposes I made up a file containing
a single 750K query (just a "SELECT big-honking-string-constant", with
the string literal broken into lines of 75 bytes).  The client-side
(psql) CPU time to run this file looks about like this on my machine:

			  PGCLIENTENCODING
			UNICODE		SJIS

CVS tip			1.57		1.82

flex implementation	0.93		2.33

The flex implementation is consistently faster than CVS tip when dealing
with backend-compatible encodings (such as UTF-8).  It's consistently
slower when it has to deal with a non-backend-safe encoding such as SJIS
or Big5.  But for real-world cases the differential is down in the noise
either way.

I'm inclined to apply this but I can see where a person not comfortable
with flex might feel differently.  Opinions?

			regards, tom lane


Attachment: psql-flex.patch.gz
Description: application/octet-stream (12.8 KB)

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Subject: Re: Doing psql's lexing with flex
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