PostGIS  2.1.10dev-r@@SVN_REVISION@@
static float8 estimate_selectivity ( const GBOX box,
const ND_STATS nd_stats,
int  mode 
)
static

This function returns an estimate of the selectivity of a search GBOX by looking at data in the ND_STATS structure.

The selectivity is a float from 0-1, that estimates the proportion of the rows in the table that will be returned as a result of the search box.

To get our estimate, we need "only" sum up the values * the proportion of each cell in the histogram that falls within the search box, then divide by the number of features that generated the histogram.

Definition at line 1776 of file gserialized_estimate.c.

References ND_STATS_T::extent, FALLBACK_ND_SEL, gbox_ndims(), ND_STATS_T::histogram_cells, ND_STATS_T::histogram_features, ND_BOX_T::max, ND_BOX_T::min, ND_IBOX_T::min, nd_box_contains(), nd_box_from_gbox(), nd_box_intersects(), nd_box_overlap(), nd_box_ratio(), nd_box_to_json(), ND_DIMS, nd_increment(), nd_stats_value_index(), ND_STATS_T::ndims, ND_STATS_T::size, and ND_STATS_T::value.

Referenced by _postgis_gserialized_sel(), and gserialized_gist_sel().

1777 {
1778  int d; /* counter */
1779  float8 selectivity;
1780  ND_BOX nd_box;
1781  ND_IBOX nd_ibox;
1782  int at[ND_DIMS];
1783  double cell_size[ND_DIMS];
1784  double min[ND_DIMS];
1785  double max[ND_DIMS];
1786  double total_count = 0.0;
1787  int ndims_max = Max(nd_stats->ndims, gbox_ndims(box));
1788 // int ndims_min = Min(nd_stats->ndims, gbox_ndims(box));
1789 
1790  /* Calculate the overlap of the box on the histogram */
1791  if ( ! nd_stats )
1792  {
1793  elog(NOTICE, " estimate_selectivity called with null input");
1794  return FALLBACK_ND_SEL;
1795  }
1796 
1797  /* Initialize nd_box. */
1798  nd_box_from_gbox(box, &nd_box);
1799 
1800  /*
1801  * To return 2D stats on an ND sample, we need to make the
1802  * 2D box cover the full range of the other dimensions in the
1803  * histogram.
1804  */
1805  POSTGIS_DEBUGF(3, " mode: %d", mode);
1806  if ( mode == 2 )
1807  {
1808  POSTGIS_DEBUG(3, " in 2d mode, stripping the computation down to 2d");
1809  ndims_max = 2;
1810  }
1811 
1812  POSTGIS_DEBUGF(3, " nd_stats->extent: %s", nd_box_to_json(&(nd_stats->extent), nd_stats->ndims));
1813  POSTGIS_DEBUGF(3, " nd_box: %s", nd_box_to_json(&(nd_box), gbox_ndims(box)));
1814 
1815  /*
1816  * Search box completely misses histogram extent?
1817  * We have to intersect in all N dimensions or else we have
1818  * zero interaction under the &&& operator. It's important
1819  * to short circuit in this case, as some of the tests below
1820  * will return junk results when run on non-intersecting inputs.
1821  */
1822  if ( ! nd_box_intersects(&nd_box, &(nd_stats->extent), ndims_max) )
1823  {
1824  POSTGIS_DEBUG(3, " search box does not overlap histogram, returning 0");
1825  return 0.0;
1826  }
1827 
1828  /* Search box completely contains histogram extent! */
1829  if ( nd_box_contains(&nd_box, &(nd_stats->extent), ndims_max) )
1830  {
1831  POSTGIS_DEBUG(3, " search box contains histogram, returning 1");
1832  return 1.0;
1833  }
1834 
1835  /* Calculate the overlap of the box on the histogram */
1836  if ( ! nd_box_overlap(nd_stats, &nd_box, &nd_ibox) )
1837  {
1838  POSTGIS_DEBUG(3, " search box overlap with stats histogram failed");
1839  return FALLBACK_ND_SEL;
1840  }
1841 
1842  /* Work out some measurements of the histogram */
1843  for ( d = 0; d < nd_stats->ndims; d++ )
1844  {
1845  /* Cell size in each dim */
1846  min[d] = nd_stats->extent.min[d];
1847  max[d] = nd_stats->extent.max[d];
1848  cell_size[d] = (max[d] - min[d]) / nd_stats->size[d];
1849  POSTGIS_DEBUGF(3, " cell_size[%d] : %.9g", d, cell_size[d]);
1850 
1851  /* Initialize the counter */
1852  at[d] = nd_ibox.min[d];
1853  }
1854 
1855  /* Move through all the overlap values and sum them */
1856  do
1857  {
1858  float cell_count, ratio;
1859  ND_BOX nd_cell;
1860 
1861  /* We have to pro-rate partially overlapped cells. */
1862  for ( d = 0; d < nd_stats->ndims; d++ )
1863  {
1864  nd_cell.min[d] = min[d] + (at[d]+0) * cell_size[d];
1865  nd_cell.max[d] = min[d] + (at[d]+1) * cell_size[d];
1866  }
1867 
1868  ratio = nd_box_ratio(&nd_box, &nd_cell, nd_stats->ndims);
1869  cell_count = nd_stats->value[nd_stats_value_index(nd_stats, at)];
1870 
1871  /* Add the pro-rated count for this cell to the overall total */
1872  total_count += cell_count * ratio;
1873  POSTGIS_DEBUGF(4, " cell (%d,%d), cell value %.6f, ratio %.6f", at[0], at[1], cell_count, ratio);
1874  }
1875  while ( nd_increment(&nd_ibox, nd_stats->ndims, at) );
1876 
1877  /* Scale by the number of features in our histogram to get the proportion */
1878  selectivity = total_count / nd_stats->histogram_features;
1879 
1880  POSTGIS_DEBUGF(3, " nd_stats->histogram_features = %f", nd_stats->histogram_features);
1881  POSTGIS_DEBUGF(3, " nd_stats->histogram_cells = %f", nd_stats->histogram_cells);
1882  POSTGIS_DEBUGF(3, " sum(overlapped histogram cells) = %f", total_count);
1883  POSTGIS_DEBUGF(3, " selectivity = %f", selectivity);
1884 
1885  /* Prevent rounding overflows */
1886  if (selectivity > 1.0) selectivity = 1.0;
1887  else if (selectivity < 0.0) selectivity = 0.0;
1888 
1889  return selectivity;
1890 }
static int nd_increment(ND_IBOX *ibox, int ndims, int *counter)
Given an n-d index array (counter), and a domain to increment it in (ibox) increment it by one...
static int nd_box_overlap(const ND_STATS *nd_stats, const ND_BOX *nd_box, ND_IBOX *nd_ibox)
What stats cells overlap with this ND_BOX? Put the lowest cell addresses in ND_IBOX->min and the high...
#define ND_DIMS
The maximum number of dimensions our code can handle.
static int nd_box_contains(const ND_BOX *a, const ND_BOX *b, int ndims)
Return TRUE if ND_BOX a contains b, false otherwise.
static void nd_box_from_gbox(const GBOX *gbox, ND_BOX *nd_box)
Set the values of an ND_BOX from a GBOX.
#define FALLBACK_ND_SEL
More modest fallback selectivity factor.
static int nd_stats_value_index(const ND_STATS *stats, int *indexes)
Given a position in the n-d histogram (i,j,k) return the position in the 1-d values array...
float4 size[ND_DIMS]
static int nd_box_intersects(const ND_BOX *a, const ND_BOX *b, int ndims)
Return TRUE if ND_BOX a overlaps b, false otherwise.
int min[ND_DIMS]
N-dimensional box index type.
static char * nd_box_to_json(const ND_BOX *nd_box, int ndims)
Convert an ND_BOX to a JSON string for printing.
float4 max[ND_DIMS]
float4 min[ND_DIMS]
static double nd_box_ratio(const ND_BOX *b1, const ND_BOX *b2, int ndims)
Returns the proportion of b2 that is covered by b1.
N-dimensional box type for calculations, to avoid doing explicit axis conversions from GBOX in all ca...
static int gbox_ndims(const GBOX *gbox)
Given that geodetic boxes are X/Y/Z regardless of the underlying geometry dimensionality and other bo...

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