On 08/25/2011 01:16 PM, Alvaro Herrera wrote:
Excerpts from Tom Lane's message of jue ago 25 14:00:57 -0300 2011:
Robert Haas<robertmh...@gmail.com> writes:
On Thu, Aug 25, 2011 at 12:47 PM, Tom Lane<t...@sss.pgh.pa.us> wrote:
1. Somebody broke the no-backtracking property back in 9.0 while adding
quoted variable substitution. According to the flex manual, use of
backtracking creates a performance penalty. We once measured the
backend's lexer as being about a third faster with backtrack avoidance,
and presumably it's about the same for psql's. This is not hard to fix,
but should I consider it a bug fix and back-patch? We've not had
complaints about psql getting slower as of 9.0.
That may well have been me.
[ checks "git blame" ] Well, you commmitted the patch anyway: d0cfc018.
How would I have known that I broke it?
Per the header comments in the backend lexer, you should run flex with
"-b" switch and verify that the resulting lex.backup file says "no
backing up". I've occasionally thought about automating that, but I'm
not sure if the output is entirely locale- and flex-version-independent.
It is locale dependent, though of course for the automated check you
could just run flex under LC_ALL=C.
$ /usr/bin/flex -Cfe -b /pgsql/source/REL8_4_STABLE/src/bin/psql/psqlscan.l
$ cat lex.backup
Sin retroceso.
$ LC_ALL=C /usr/bin/flex -Cfe -b
/pgsql/source/REL8_4_STABLE/src/bin/psql/psqlscan.l
$ cat lex.backup
No backing up.
We could just add -b unconditionally to the flex flags and then count
the number of lines in lex.backup. If it's greater that 1 whine loudly,
or even fail, otherwise remove lex.backup. Would that avoid locale
dependencies?
cheers
andrew
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