Re: Large arrays give long lag on server side before command executes

On 25. sep 2004, at 01:42, Tom Lane wrote:

> Steve Crawford <scrawford(at)pinpointresearch(dot)com> writes:
>> On Friday 24 September 2004 7:32 am, Tom Lane wrote:
>>> I'm betting on some O(N^2) behavior in the array code, but it'll be
>>> difficult to pinpoint without profile results.
>
>> Possibly the bug we discussed early last year (IIRC added to todo but
>> not fixed):
>> http://archives.postgresql.org/pgsql-performance/2003-01/msg00235.php
>
> No, it's not the same --- David sent me some results off-list and it
> seems ReadArrayStr is the culprit. I'm not completely sure why yet.

After playing around, I found the culprit. As always, I might just as
well have gone to the code. Too bad C always reads to me like an old
manuscript. I know the language, but I have to stutter through it.
Anyway, these lines should trigger anyones big-O alarm:

/* adt/arrayfuncs.c, line 575ff in v. 7.4.1. In the big switch
statement inside ReadArrayStr() */

case '\\':
{
char *cptr;

/* Crunch the string on top of the backslash. */
for (cptr = ptr; *cptr != '\0'; cptr++) /* BAD :) */
*cptr = *(cptr + 1);
if (*ptr == '\0')
ereport(ERROR,
(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
errmsg("malformed array literal: \"%s\"", arrayStr)));
break;
}
case '\"':
{
char *cptr;

scanning_string = !scanning_string;
/* Crunch the string on top of the quote. */
for (cptr = ptr; *cptr != '\0'; cptr++) /* BAD :) */
*cptr = *(cptr + 1);
/* Back up to not miss following character. */
ptr--;
break;
}

I'm toying with a reimplementation of this algorithm, but it might be
some days before I have the time. Didn't look at the 8.0 beta yet, so
of course someone might have been there already.

I'll be back when I have more.

d.

> FWIW, the printtup part of your gripe is fixed in CVS tip.
>
> regards, tom lane
>
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--
David Helgason,
Business Development et al.,
Over the Edge I/S (http://otee.dk)
Direct line +45 2620 0663
Main line +45 3264 5049

--
David Helgason,
Business Development et al.,
Over the Edge I/S (http://otee.dk)
Direct line +45 2620 0663
Main line +45 3264 5049