Re: Performance patch for Win32

I am hesitant to write a function like AllocateDirWithFilePattern
if the pattern is simply ignored on non-Windows.  In my patch,
the pattern underspecified the files, and the ad-hoc matching code
applied to all the returned files tightened that up.  But a person
could just as well overspecify the pattern and then they would get
different behavior on Windows vs. non-Windows, with fewer
files returned by FindNextFile() than would have matched the
ad-hoc pattern.

In this particular instance, AllocateDirWithFilePrefix would
work, and could be applied on all platforms, using something
like:

    temp_dir = AllocateDirWithFilePrefix(tmpdirname,
                                        PG_TEMP_FILE_PREFIX);

and that could be converted to PG_TEMP_FILE_PREFIX "*"
on Windows, and on non-Windows the function itself could
apply the prefix check easily enough.

Is AllocateDirWithFilePrefix overly specific?  Clearly we
could also take a Suffix argument, but if we go too far down
this road we just reinvent regular expressions....

mark

________________________________
From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mark Dilger <markdilger(at)yahoo(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Sent: Tuesday, May 29, 2012 2:30 PM
Subject: Re: [HACKERS] Performance patch for Win32

Mark Dilger <markdilger(at)yahoo(dot)com> writes:
> 4) Other places in the PostgreSQL sources where directory
> iteration is needed should probably use a pattern if possible
> when running on Windows.  Thus, it might make more
> sense to have a version of ReadDir that explicitly takes a
> pattern, and use that version of ReadDir elsewhere in the
> codebase.

Yeah, I think a separate argument passed to an AllocateDir variant
would be a less ugly way to deal with this.  For example, in place
of your first #ifdef block just write

        temp_dir = AllocateDirWithFilePattern(tmpdirname,
                                              PG_TEMP_FILE_PREFIX "*");

What is not immediately obvious to me is whether we should try to make
the pattern argument do something useful on non-Windows platforms
(and thus allow removal of the ad-hoc pattern match code in the loops
where this is used); versus just treating it as a no-op on non-Windows.
If we did that, we'd have to consider that Windows gets to define what
the pattern language is and try to emulate that; which is likely to be
expensive enough to not be a win, not to mention that non-Windows
developers might not be terribly familiar with the finer points of the
pattern language.

I'm kind of inclined to consider that we should just treat the pattern
option as a Windows-specific wart and keep the ad-hoc matching code as
it is.

            regards, tom lane