mirror/postgresql
2
0
Fork 0
mirror of https://git.postgresql.org/git/postgresql.git synced 2024-11-27 07:21:09 +08:00
Code Issues Packages Projects Releases Wiki Activity

Improve performance of our private version of qsort. Per recent testing,

Browse Source 
the logic it contained to switch to insertion sort for near-sorted input was
in fact a big loss, because it could fairly easily be fooled into applying
insertion sort to large subfiles that weren't all that well ordered.  Remove
that, and instead add a simple check for already-perfectly-sorted input, as
per suggestion from Dann Corbit.  This adds at worst O(N*lgN) overhead, and
usually far less, while sometimes allowing a subfile sort to finish in O(N)
time.  Preliminary testing says this is an improvement over the basic
Bentley & McIlroy code for many nonrandom inputs, and it costs almost
nothing when the input is random.
This commit is contained in:
Tom Lane 2006-03-21 19:49:15 +00:00
parent 570b726533
commit a3f0b3d68f
1 changed files with 20 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

37
src/port/qsort.c
View File

@ -2,8 +2,10 @@
* Modifications from vanilla NetBSD source:
* Add do ... while() macro fix
* Remove __inline, _DIAGASSERTs, __P
* Remove ill-considered "swap_cnt" switch to insertion sort,
* in favor of a simple check for presorted input.
*
* $PostgreSQL: pgsql/src/port/qsort.c,v 1.8 2005/10/15 02:49:51 momjian Exp $
* $PostgreSQL: pgsql/src/port/qsort.c,v 1.9 2006/03/21 19:49:15 tgl Exp $
*/
/* $NetBSD: qsort.c,v 1.13 2003/08/07 16:43:42 agc Exp $ */
@ -47,7 +49,11 @@ static void swapfunc(char *, char *, size_t, int);
#define min(a, b) ((a) < (b) ? (a) : (b))
/*
* Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".
* Qsort routine based on J. L. Bentley and M. D. McIlroy,
* "Engineering a sort function",
* Software--Practice and Experience 23 (1993) 1249-1265.
* We have modified their original by adding a check for already-sorted input,
* which seems to be a win per discussions on pgsql-hackers around 2006-03-21.
*/
#define swapcode(TYPE, parmi, parmj, n) \
do { \
@ -116,10 +122,9 @@ int (*cmp) (const void *, const void *);
int d,
r,
swaptype,
swap_cnt;
presorted;
loop:SWAPINIT(a, es);
swap_cnt = 0;
if (n < 7)
{
for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
@ -128,6 +133,17 @@ loop:SWAPINIT(a, es);
swap(pl, pl - es);
return;
}
presorted = 1;
for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
{
if (cmp(pm - es, pm) > 0)
{
presorted = 0;
break;
}
}
if (presorted)
return;
pm = (char *) a + (n / 2) * es;
if (n > 7)
{
@ -144,7 +160,6 @@ loop:SWAPINIT(a, es);
}
swap(a, pm);
pa = pb = (char *) a + es;
pc = pd = (char *) a + (n - 1) * es;
for (;;)
{
@ -152,7 +167,6 @@ loop:SWAPINIT(a, es);
{
if (r == 0)
{
swap_cnt = 1;
swap(pa, pb);
pa += es;
}
@ -162,7 +176,6 @@ loop:SWAPINIT(a, es);
{
if (r == 0)
{
swap_cnt = 1;
swap(pc, pd);
pd -= es;
}
@ -171,19 +184,9 @@ loop:SWAPINIT(a, es);
if (pb > pc)
break;
swap(pb, pc);
swap_cnt = 1;
pb += es;
pc -= es;
}
if (swap_cnt == 0)
{ /* Switch to insertion sort */
for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
for (pl = pm; pl > (char *) a && cmp(pl - es, pl) > 0;
pl -= es)
swap(pl, pl - es);
return;
}
pn = (char *) a + n * es;
r = min(pa - (char *) a, pb - pa);
vecswap(a, pb - r, r);