extern int seg_yydebug;
*/
+/*
+ * Auxiliary data structure for picksplit method.
+ */
+typedef struct
+{
+ float center;
+ OffsetNumber index;
+ SEG *data;
+} gseg_picksplit_item;
+
/*
** Input/Output routines
*/
return (result);
}
+/*
+ * Compare function for gseg_picksplit_item: sort by center.
+ */
+static int
+gseg_picksplit_item_cmp(const void *a, const void *b)
+{
+ const gseg_picksplit_item *i1 = (const gseg_picksplit_item *) a;
+ const gseg_picksplit_item *i2 = (const gseg_picksplit_item *) b;
+ if (i1->center < i2->center)
+ return -1;
+ else if (i1->center == i2->center)
+ return 0;
+ else
+ return 1;
+}
/*
-** The GiST PickSplit method for segments
-** We use Guttman's poly time split algorithm
-*/
+ * The GiST PickSplit method for segments
+ *
+ * We used to use Guttman's split algorithm here, but since the data is 1-D
+ * it's easier and more robust to just sort the segments by center-point and
+ * split at the middle.
+ */
GIST_SPLITVEC *
gseg_picksplit(GistEntryVector *entryvec,
GIST_SPLITVEC *v)
{
- OffsetNumber i,
- j;
- SEG *datum_alpha,
- *datum_beta;
+ int i;
SEG *datum_l,
- *datum_r;
- SEG *union_d,
- *union_dl,
- *union_dr;
- SEG *inter_d;
- bool firsttime;
- float size_alpha,
- size_beta,
- size_union,
- size_inter;
- float size_waste,
- waste;
- float size_l,
- size_r;
- int nbytes;
- OffsetNumber seed_1 = 1,
- seed_2 = 2;
+ *datum_r,
+ *seg;
+ gseg_picksplit_item *sort_items;
OffsetNumber *left,
*right;
OffsetNumber maxoff;
+ OffsetNumber firstright;
#ifdef GIST_DEBUG
fprintf(stderr, "picksplit\n");
#endif
- maxoff = entryvec->n - 2;
- nbytes = (maxoff + 2) * sizeof(OffsetNumber);
- v->spl_left = (OffsetNumber *) palloc(nbytes);
- v->spl_right = (OffsetNumber *) palloc(nbytes);
+ /* Valid items in entryvec->vector[] are indexed 1..maxoff */
+ maxoff = entryvec->n - 1;
- firsttime = true;
- waste = 0.0;
-
- for (i = FirstOffsetNumber; i < maxoff; i = OffsetNumberNext(i))
+ /*
+ * Prepare the auxiliary array and sort it.
+ */
+ sort_items = (gseg_picksplit_item *)
+ palloc(maxoff * sizeof(gseg_picksplit_item));
+ for (i = 1; i <= maxoff; i++)
{
- datum_alpha = (SEG *) DatumGetPointer(entryvec->vector[i].key);
- for (j = OffsetNumberNext(i); j <= maxoff; j = OffsetNumberNext(j))
- {
- datum_beta = (SEG *) DatumGetPointer(entryvec->vector[j].key);
-
- /* compute the wasted space by unioning these guys */
- /* size_waste = size_union - size_inter; */
- union_d = seg_union(datum_alpha, datum_beta);
- rt_seg_size(union_d, &size_union);
- inter_d = seg_inter(datum_alpha, datum_beta);
- rt_seg_size(inter_d, &size_inter);
- size_waste = size_union - size_inter;
-
- /*
- * are these a more promising split that what we've already seen?
- */
- if (size_waste > waste || firsttime)
- {
- waste = size_waste;
- seed_1 = i;
- seed_2 = j;
- firsttime = false;
- }
- }
+ seg = (SEG *) DatumGetPointer(entryvec->vector[i].key);
+ /* center calculation is done this way to avoid possible overflow */
+ sort_items[i - 1].center = seg->lower*0.5f + seg->upper*0.5f;
+ sort_items[i - 1].index = i;
+ sort_items[i - 1].data = seg;
}
+ qsort(sort_items, maxoff, sizeof(gseg_picksplit_item),
+ gseg_picksplit_item_cmp);
+ /* sort items below "firstright" will go into the left side */
+ firstright = maxoff / 2;
+
+ v->spl_left = (OffsetNumber *) palloc(maxoff * sizeof(OffsetNumber));
+ v->spl_right = (OffsetNumber *) palloc(maxoff * sizeof(OffsetNumber));
left = v->spl_left;
v->spl_nleft = 0;
right = v->spl_right;
v->spl_nright = 0;
- datum_alpha = (SEG *) DatumGetPointer(entryvec->vector[seed_1].key);
- datum_l = seg_union(datum_alpha, datum_alpha);
- rt_seg_size(datum_l, &size_l);
- datum_beta = (SEG *) DatumGetPointer(entryvec->vector[seed_2].key);
- datum_r = seg_union(datum_beta, datum_beta);
- rt_seg_size(datum_r, &size_r);
-
/*
- * Now split up the regions between the two seeds. An important property
- * of this split algorithm is that the split vector v has the indices of
- * items to be split in order in its left and right vectors. We exploit
- * this property by doing a merge in the code that actually splits the
- * page.
- *
- * For efficiency, we also place the new index tuple in this loop. This is
- * handled at the very end, when we have placed all the existing tuples
- * and i == maxoff + 1.
+ * Emit segments to the left output page, and compute its bounding box.
*/
-
- maxoff = OffsetNumberNext(maxoff);
- for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
+ datum_l = (SEG *) palloc(sizeof(SEG));
+ memcpy(datum_l, sort_items[0].data, sizeof(SEG));
+ *left++ = sort_items[0].index;
+ v->spl_nleft++;
+ for (i = 1; i < firstright; i++)
{
- /*
- * If we've already decided where to place this item, just put it on
- * the right list. Otherwise, we need to figure out which page needs
- * the least enlargement in order to store the item.
- */
-
- if (i == seed_1)
- {
- *left++ = i;
- v->spl_nleft++;
- continue;
- }
- else if (i == seed_2)
- {
- *right++ = i;
- v->spl_nright++;
- continue;
- }
-
- /* okay, which page needs least enlargement? */
- datum_alpha = (SEG *) DatumGetPointer(entryvec->vector[i].key);
- union_dl = seg_union(datum_l, datum_alpha);
- union_dr = seg_union(datum_r, datum_alpha);
- rt_seg_size(union_dl, &size_alpha);
- rt_seg_size(union_dr, &size_beta);
+ datum_l = seg_union(datum_l, sort_items[i].data);
+ *left++ = sort_items[i].index;
+ v->spl_nleft++;
+ }
- /* pick which page to add it to */
- if (size_alpha - size_l < size_beta - size_r)
- {
- datum_l = union_dl;
- size_l = size_alpha;
- *left++ = i;
- v->spl_nleft++;
- }
- else
- {
- datum_r = union_dr;
- size_r = size_alpha;
- *right++ = i;
- v->spl_nright++;
- }
+ /*
+ * Likewise for the right page.
+ */
+ datum_r = (SEG *) palloc(sizeof(SEG));
+ memcpy(datum_r, sort_items[firstright].data, sizeof(SEG));
+ *right++ = sort_items[firstright].index;
+ v->spl_nright++;
+ for (i = firstright + 1; i < maxoff; i++)
+ {
+ datum_r = seg_union(datum_r, sort_items[i].data);
+ *right++ = sort_items[i].index;
+ v->spl_nright++;
}
- *left = *right = FirstOffsetNumber; /* sentinel value, see dosplit() */
v->spl_ldatum = PointerGetDatum(datum_l);
v->spl_rdatum = PointerGetDatum(datum_r);
projects / postgresql / commitdiff