pg_next_dst_boundary optimization

While profiling loading a table with timestamp columns with COPY, I
spotted that a lot of time is spent in pg_next_dst_boundary. It scans a
sorted array of time segments to find the one that the given timestamp
falls within. That's slow when the array is big; GB timezone for example
has > 200 segments.

Attached is a patch to use binary search instead of linear search. On my
laptop, it reduces the time required to load million rows to this test
table:

CREATE TABLE timestamptable (
  id int,
  ts1 timestamp,
  ts2 timestamp,
  ts3 timestamp,
  ts4 timestamp
);

from ~16.4s to ~11.9s.

It would be nice to slip this into 8.3...

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com
Index: src/timezone/localtime.c
===================================================================
RCS file: /home/hlinnaka/pgcvsrepository/pgsql/src/timezone/localtime.c,v
retrieving revision 1.17
diff -c -r1.17 localtime.c
*** src/timezone/localtime.c    4 Aug 2007 19:29:25 -0000    1.17
--- src/timezone/localtime.c    18 Sep 2007 13:41:03 -0000
***************
*** 1008,1013 ****
--- 1008,1014 ----
      int            i;
      int            j;
      const pg_time_t t = *timep;
+     int high, low;

      sp = &tz->state;
      if (sp->timecnt == 0)
***************
*** 1056,1064 ****
          return 1;
      }
      /* Else search to find the containing segment */
!     for (i = 1; i < sp->timecnt; ++i)
!         if (t <= sp->ats[i])
!             break;
      j = sp->types[i - 1];
      ttisp = &sp->ttis[j];
      *before_gmtoff = ttisp->tt_gmtoff;
--- 1057,1074 ----
          return 1;
      }
      /* Else search to find the containing segment */
!     low = 1;
!     high = sp->timecnt - 1;
!     while (high > low)
!     {
!         int mid = low + ((high - low) / 2);
!         if (t > sp->ats[mid])
!             low = mid + 1;
!         else
!             high = mid;
!     }
!     i = high;
!
      j = sp->types[i - 1];
      ttisp = &sp->ttis[j];
      *before_gmtoff = ttisp->tt_gmtoff;