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

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
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37
src/port/qsort.c
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@ -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);