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Commit2a6ebe7

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Fix contrib/seg's GiST picksplit method.
This patch replaces Guttman's generalized split method with a simplesort-by-center-points algorithm. Since the data is only one-dimensionalwe don't really need the slow and none-too-stable Guttman method.This is in part a bug fix, since seg has the same size_alpha versussize_beta typo that was recently fixed in contrib/cube. It seemsprudent to apply this rather aggressive fix only in HEAD, though.Back branches will just get the typo fix.Alexander Korotkov, reviewed by Yeb Havinga
1 parent290f160 commit2a6ebe7

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-116
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1 file changed

+78
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‎contrib/seg/seg.c

Lines changed: 78 additions & 116 deletions
Original file line numberDiff line numberDiff line change
@@ -33,6 +33,16 @@ extern void seg_scanner_finish(void);
3333
extern int seg_yydebug;
3434
*/
3535

36+
/*
37+
* Auxiliary data structure for picksplit method.
38+
*/
39+
typedefstruct
40+
{
41+
floatcenter;
42+
OffsetNumberindex;
43+
SEG*data;
44+
}gseg_picksplit_item;
45+
3646
/*
3747
** Input/Output routines
3848
*/
@@ -292,152 +302,104 @@ gseg_penalty(GISTENTRY *origentry, GISTENTRY *newentry, float *result)
292302
return (result);
293303
}
294304

305+
/*
306+
* Compare function for gseg_picksplit_item: sort by center.
307+
*/
308+
staticint
309+
gseg_picksplit_item_cmp(constvoid*a,constvoid*b)
310+
{
311+
constgseg_picksplit_item*i1= (constgseg_picksplit_item*)a;
312+
constgseg_picksplit_item*i2= (constgseg_picksplit_item*)b;
295313

314+
if (i1->center<i2->center)
315+
return-1;
316+
elseif (i1->center==i2->center)
317+
return0;
318+
else
319+
return1;
320+
}
296321

297322
/*
298-
** The GiST PickSplit method for segments
299-
** We use Guttman's poly time split algorithm
300-
*/
323+
* The GiST PickSplit method for segments
324+
*
325+
* We used to use Guttman's split algorithm here, but since the data is 1-D
326+
* it's easier and more robust to just sort the segments by center-point and
327+
* split at the middle.
328+
*/
301329
GIST_SPLITVEC*
302330
gseg_picksplit(GistEntryVector*entryvec,
303331
GIST_SPLITVEC*v)
304332
{
305-
OffsetNumberi,
306-
j;
307-
SEG*datum_alpha,
308-
*datum_beta;
333+
inti;
309334
SEG*datum_l,
310-
*datum_r;
311-
SEG*union_d,
312-
*union_dl,
313-
*union_dr;
314-
SEG*inter_d;
315-
boolfirsttime;
316-
floatsize_alpha,
317-
size_beta,
318-
size_union,
319-
size_inter;
320-
floatsize_waste,
321-
waste;
322-
floatsize_l,
323-
size_r;
324-
intnbytes;
325-
OffsetNumberseed_1=1,
326-
seed_2=2;
335+
*datum_r,
336+
*seg;
337+
gseg_picksplit_item*sort_items;
327338
OffsetNumber*left,
328339
*right;
329340
OffsetNumbermaxoff;
341+
OffsetNumberfirstright;
330342

331343
#ifdefGIST_DEBUG
332344
fprintf(stderr,"picksplit\n");
333345
#endif
334346

335-
maxoff=entryvec->n-2;
336-
nbytes= (maxoff+2)*sizeof(OffsetNumber);
337-
v->spl_left= (OffsetNumber*)palloc(nbytes);
338-
v->spl_right= (OffsetNumber*)palloc(nbytes);
347+
/* Valid items in entryvec->vector[] are indexed 1..maxoff */
348+
maxoff=entryvec->n-1;
339349

340-
firsttime= true;
341-
waste=0.0;
342-
343-
for (i=FirstOffsetNumber;i<maxoff;i=OffsetNumberNext(i))
350+
/*
351+
* Prepare the auxiliary array and sort it.
352+
*/
353+
sort_items= (gseg_picksplit_item*)
354+
palloc(maxoff*sizeof(gseg_picksplit_item));
355+
for (i=1;i <=maxoff;i++)
344356
{
345-
datum_alpha= (SEG*)DatumGetPointer(entryvec->vector[i].key);
346-
for (j=OffsetNumberNext(i);j <=maxoff;j=OffsetNumberNext(j))
347-
{
348-
datum_beta= (SEG*)DatumGetPointer(entryvec->vector[j].key);
349-
350-
/* compute the wasted space by unioning these guys */
351-
/* size_waste = size_union - size_inter; */
352-
union_d=seg_union(datum_alpha,datum_beta);
353-
rt_seg_size(union_d,&size_union);
354-
inter_d=seg_inter(datum_alpha,datum_beta);
355-
rt_seg_size(inter_d,&size_inter);
356-
size_waste=size_union-size_inter;
357-
358-
/*
359-
* are these a more promising split that what we've already seen?
360-
*/
361-
if (size_waste>waste||firsttime)
362-
{
363-
waste=size_waste;
364-
seed_1=i;
365-
seed_2=j;
366-
firsttime= false;
367-
}
368-
}
357+
seg= (SEG*)DatumGetPointer(entryvec->vector[i].key);
358+
/* center calculation is done this way to avoid possible overflow */
359+
sort_items[i-1].center=seg->lower*0.5f+seg->upper*0.5f;
360+
sort_items[i-1].index=i;
361+
sort_items[i-1].data=seg;
369362
}
363+
qsort(sort_items,maxoff,sizeof(gseg_picksplit_item),
364+
gseg_picksplit_item_cmp);
370365

366+
/* sort items below "firstright" will go into the left side */
367+
firstright=maxoff /2;
368+
369+
v->spl_left= (OffsetNumber*)palloc(maxoff*sizeof(OffsetNumber));
370+
v->spl_right= (OffsetNumber*)palloc(maxoff*sizeof(OffsetNumber));
371371
left=v->spl_left;
372372
v->spl_nleft=0;
373373
right=v->spl_right;
374374
v->spl_nright=0;
375375

376-
datum_alpha= (SEG*)DatumGetPointer(entryvec->vector[seed_1].key);
377-
datum_l=seg_union(datum_alpha,datum_alpha);
378-
rt_seg_size(datum_l,&size_l);
379-
datum_beta= (SEG*)DatumGetPointer(entryvec->vector[seed_2].key);
380-
datum_r=seg_union(datum_beta,datum_beta);
381-
rt_seg_size(datum_r,&size_r);
382-
383376
/*
384-
* Now split up the regions between the two seeds.An important property
385-
* of this split algorithm is that the split vector v has the indices of
386-
* items to be split in order in its left and right vectors. We exploit
387-
* this property by doing a merge in the code that actually splits the
388-
* page.
389-
*
390-
* For efficiency, we also place the new index tuple in this loop. This is
391-
* handled at the very end, when we have placed all the existing tuples
392-
* and i == maxoff + 1.
377+
* Emit segments to the left output page, and compute its bounding box.
393378
*/
394-
395-
maxoff=OffsetNumberNext(maxoff);
396-
for (i=FirstOffsetNumber;i <=maxoff;i=OffsetNumberNext(i))
379+
datum_l= (SEG*)palloc(sizeof(SEG));
380+
memcpy(datum_l,sort_items[0].data,sizeof(SEG));
381+
*left++=sort_items[0].index;
382+
v->spl_nleft++;
383+
for (i=1;i<firstright;i++)
397384
{
398-
/*
399-
* If we've already decided where to place this item, just put it on
400-
* the right list.Otherwise, we need to figure out which page needs
401-
* the least enlargement in order to store the item.
402-
*/
403-
404-
if (i==seed_1)
405-
{
406-
*left++=i;
407-
v->spl_nleft++;
408-
continue;
409-
}
410-
elseif (i==seed_2)
411-
{
412-
*right++=i;
413-
v->spl_nright++;
414-
continue;
415-
}
416-
417-
/* okay, which page needs least enlargement? */
418-
datum_alpha= (SEG*)DatumGetPointer(entryvec->vector[i].key);
419-
union_dl=seg_union(datum_l,datum_alpha);
420-
union_dr=seg_union(datum_r,datum_alpha);
421-
rt_seg_size(union_dl,&size_alpha);
422-
rt_seg_size(union_dr,&size_beta);
385+
datum_l=seg_union(datum_l,sort_items[i].data);
386+
*left++=sort_items[i].index;
387+
v->spl_nleft++;
388+
}
423389

424-
/* pick which page to add it to */
425-
if (size_alpha-size_l<size_beta-size_r)
426-
{
427-
datum_l=union_dl;
428-
size_l=size_alpha;
429-
*left++=i;
430-
v->spl_nleft++;
431-
}
432-
else
433-
{
434-
datum_r=union_dr;
435-
size_r=size_alpha;
436-
*right++=i;
437-
v->spl_nright++;
438-
}
390+
/*
391+
* Likewise for the right page.
392+
*/
393+
datum_r= (SEG*)palloc(sizeof(SEG));
394+
memcpy(datum_r,sort_items[firstright].data,sizeof(SEG));
395+
*right++=sort_items[firstright].index;
396+
v->spl_nright++;
397+
for (i=firstright+1;i<maxoff;i++)
398+
{
399+
datum_r=seg_union(datum_r,sort_items[i].data);
400+
*right++=sort_items[i].index;
401+
v->spl_nright++;
439402
}
440-
*left=*right=FirstOffsetNumber;/* sentinel value, see dosplit() */
441403

442404
v->spl_ldatum=PointerGetDatum(datum_l);
443405
v->spl_rdatum=PointerGetDatum(datum_r);

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