g_box_consider_split()

static void g_box_consider_split ( ConsiderSplitContext *  context, int  dimNum, float  rightLower, int  minLeftCount, float  leftUpper, int  maxLeftCount  )

inlinestatic

Definition at line 1507 of file gserialized_gist_2d.c.

{
        int                     leftCount,
                                rightCount;
        float4          ratio,
                                overlap;
        float           range;
 
        POSTGIS_DEBUGF(5, "consider split: dimNum = %d, rightLower = %f, "
                "minLeftCount = %d, leftUpper = %f, maxLeftCount = %d ",
                dimNum, rightLower, minLeftCount, leftUpper, maxLeftCount);
 
        /*
         * Calculate entries distribution ratio assuming most uniform distribution
         * of common entries.
         */
        if (minLeftCount >= (context->entriesCount + 1) / 2)
        {
                leftCount = minLeftCount;
        }
        else
        {
                if (maxLeftCount <= context->entriesCount / 2)
                        leftCount = maxLeftCount;
                else
                        leftCount = context->entriesCount / 2;
        }
        rightCount = context->entriesCount - leftCount;
 
        /*
         * Ratio of split - quotient between size of lesser group and total
         * entries count.
         */
        ratio = ((float4) Min(leftCount, rightCount)) /
                ((float4) context->entriesCount);
 
        if (ratio > LIMIT_RATIO)
        {
                bool            selectthis = false;
 
                /*
                 * The ratio is acceptable, so compare current split with previously
                 * selected one. Between splits of one dimension we search for minimal
                 * overlap (allowing negative values) and minimal ration (between same
                 * overlaps. We switch dimension if find less overlap (non-negative)
                 * or less range with same overlap.
                 */
                if (dimNum == 0)
                        range = context->boundingBox.xmax - context->boundingBox.xmin;
                else
                        range = context->boundingBox.ymax - context->boundingBox.ymin;
 
                overlap = (leftUpper - rightLower) / range;
 
                /* If there is no previous selection, select this */
                if (context->first)
                        selectthis = true;
                else if (context->dim == dimNum)
                {
                        /*
                         * Within the same dimension, choose the new split if it has a
                         * smaller overlap, or same overlap but better ratio.
                         */
                        if (overlap < context->overlap ||
                                (overlap == context->overlap && ratio > context->ratio))
                                selectthis = true;
                }
                else
                {
                        /*
                         * Across dimensions, choose the new split if it has a smaller
                         * *non-negative* overlap, or same *non-negative* overlap but
                         * bigger range. This condition differs from the one described in
                         * the article. On the datasets where leaf MBRs don't overlap
                         * themselves, non-overlapping splits (i.e. splits which have zero
                         * *non-negative* overlap) are frequently possible. In this case
                         * splits tends to be along one dimension, because most distant
                         * non-overlapping splits (i.e. having lowest negative overlap)
                         * appears to be in the same dimension as in the previous split.
                         * Therefore MBRs appear to be very prolonged along another
                         * dimension, which leads to bad search performance. Using range
                         * as the second split criteria makes MBRs more quadratic. Using
                         * *non-negative* overlap instead of overlap as the first split
                         * criteria gives to range criteria a chance to matter, because
                         * non-overlapping splits are equivalent in this criteria.
                         */
                        if (non_negative(overlap) < non_negative(context->overlap) ||
                                (range > context->range &&
                                 non_negative(overlap) <= non_negative(context->overlap)))
                                selectthis = true;
                }
 
                if (selectthis)
                {
                        /* save information about selected split */
                        context->first = false;
                        context->ratio = ratio;
                        context->range = range;
                        context->overlap = overlap;
                        context->rightLower = rightLower;
                        context->leftUpper = leftUpper;
                        context->dim = dimNum;
                        POSTGIS_DEBUG(5, "split selected");
                }
        }
}

References ConsiderSplitContext::boundingBox, ConsiderSplitContext::dim, ConsiderSplitContext::entriesCount, ConsiderSplitContext::first, ConsiderSplitContext::leftUpper, LIMIT_RATIO, non_negative(), ConsiderSplitContext::overlap, ConsiderSplitContext::range, ConsiderSplitContext::ratio, and ConsiderSplitContext::rightLower.

Referenced by gserialized_gist_picksplit_2d().

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