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Commita3f0b3d

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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 wasin fact a big loss, because it could fairly easily be fooled into applyinginsertion sort to large subfiles that weren't all that well ordered. Removethat, and instead add a simple check for already-perfectly-sorted input, asper suggestion from Dann Corbit. This adds at worst O(N*lgN) overhead, andusually far less, while sometimes allowing a subfile sort to finish in O(N)time. Preliminary testing says this is an improvement over the basicBentley & McIlroy code for many nonrandom inputs, and it costs almostnothing when the input is random.
1 parent570b726 commita3f0b3d

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‎src/port/qsort.c‎

Lines changed: 20 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -2,8 +2,10 @@
22
*Modifications from vanilla NetBSD source:
33
* Add do ... while() macro fix
44
* Remove __inline, _DIAGASSERTs, __P
5+
* Remove ill-considered "swap_cnt" switch to insertion sort,
6+
* in favor of a simple check for presorted input.
57
*
6-
*$PostgreSQL: pgsql/src/port/qsort.c,v 1.8 2005/10/15 02:49:51 momjian Exp $
8+
*$PostgreSQL: pgsql/src/port/qsort.c,v 1.9 2006/03/21 19:49:15 tgl Exp $
79
*/
810

911
/*$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);
4749
#definemin(a,b)((a) < (b) ? (a) : (b))
4850

4951
/*
50-
* Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".
52+
* Qsort routine based on J. L. Bentley and M. D. McIlroy,
53+
* "Engineering a sort function",
54+
* Software--Practice and Experience 23 (1993) 1249-1265.
55+
* We have modified their original by adding a check for already-sorted input,
56+
* which seems to be a win per discussions on pgsql-hackers around 2006-03-21.
5157
*/
5258
#defineswapcode(TYPE,parmi,parmj,n) \
5359
do {\
@@ -116,10 +122,9 @@ int(*cmp) (const void *, const void *);
116122
intd,
117123
r,
118124
swaptype,
119-
swap_cnt;
125+
presorted;
120126

121127
loop:SWAPINIT(a,es);
122-
swap_cnt=0;
123128
if (n<7)
124129
{
125130
for (pm= (char*)a+es;pm< (char*)a+n*es;pm+=es)
@@ -128,6 +133,17 @@ loop:SWAPINIT(a, es);
128133
swap(pl,pl-es);
129134
return;
130135
}
136+
presorted=1;
137+
for (pm= (char*)a+es;pm< (char*)a+n*es;pm+=es)
138+
{
139+
if (cmp(pm-es,pm)>0)
140+
{
141+
presorted=0;
142+
break;
143+
}
144+
}
145+
if (presorted)
146+
return;
131147
pm= (char*)a+ (n /2)*es;
132148
if (n>7)
133149
{
@@ -144,15 +160,13 @@ loop:SWAPINIT(a, es);
144160
}
145161
swap(a,pm);
146162
pa=pb= (char*)a+es;
147-
148163
pc=pd= (char*)a+ (n-1)*es;
149164
for (;;)
150165
{
151166
while (pb <=pc&& (r=cmp(pb,a)) <=0)
152167
{
153168
if (r==0)
154169
{
155-
swap_cnt=1;
156170
swap(pa,pb);
157171
pa+=es;
158172
}
@@ -162,7 +176,6 @@ loop:SWAPINIT(a, es);
162176
{
163177
if (r==0)
164178
{
165-
swap_cnt=1;
166179
swap(pc,pd);
167180
pd-=es;
168181
}
@@ -171,19 +184,9 @@ loop:SWAPINIT(a, es);
171184
if (pb>pc)
172185
break;
173186
swap(pb,pc);
174-
swap_cnt=1;
175187
pb+=es;
176188
pc-=es;
177189
}
178-
if (swap_cnt==0)
179-
{/* Switch to insertion sort */
180-
for (pm= (char*)a+es;pm< (char*)a+n*es;pm+=es)
181-
for (pl=pm;pl> (char*)a&&cmp(pl-es,pl)>0;
182-
pl-=es)
183-
swap(pl,pl-es);
184-
return;
185-
}
186-
187190
pn= (char*)a+n*es;
188191
r=min(pa- (char*)a,pb-pa);
189192
vecswap(a,pb-r,r);

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