PostGIS  2.3.8dev-r@@SVN_REVISION@@

◆ estimate_selectivity()

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 1868 of file gserialized_estimate.c.

References _postgis_gserialized_stats(), 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, PG_FUNCTION_INFO_V1(), ND_STATS_T::size, and ND_STATS_T::value.

Referenced by _postgis_gserialized_sel(), and gserialized_gist_sel().

1869 {
1870  int d; /* counter */
1871  float8 selectivity;
1872  ND_BOX nd_box;
1873  ND_IBOX nd_ibox;
1874  int at[ND_DIMS];
1875  double cell_size[ND_DIMS];
1876  double min[ND_DIMS];
1877  double max[ND_DIMS];
1878  double total_count = 0.0;
1879  int ndims_max = Max(nd_stats->ndims, gbox_ndims(box));
1880 // int ndims_min = Min(nd_stats->ndims, gbox_ndims(box));
1881 
1882  /* Calculate the overlap of the box on the histogram */
1883  if ( ! nd_stats )
1884  {
1885  elog(NOTICE, " estimate_selectivity called with null input");
1886  return FALLBACK_ND_SEL;
1887  }
1888 
1889  /* Initialize nd_box. */
1890  nd_box_from_gbox(box, &nd_box);
1891 
1892  /*
1893  * To return 2D stats on an ND sample, we need to make the
1894  * 2D box cover the full range of the other dimensions in the
1895  * histogram.
1896  */
1897  POSTGIS_DEBUGF(3, " mode: %d", mode);
1898  if ( mode == 2 )
1899  {
1900  POSTGIS_DEBUG(3, " in 2d mode, stripping the computation down to 2d");
1901  ndims_max = 2;
1902  }
1903 
1904  POSTGIS_DEBUGF(3, " nd_stats->extent: %s", nd_box_to_json(&(nd_stats->extent), nd_stats->ndims));
1905  POSTGIS_DEBUGF(3, " nd_box: %s", nd_box_to_json(&(nd_box), gbox_ndims(box)));
1906 
1907  /*
1908  * Search box completely misses histogram extent?
1909  * We have to intersect in all N dimensions or else we have
1910  * zero interaction under the &&& operator. It's important
1911  * to short circuit in this case, as some of the tests below
1912  * will return junk results when run on non-intersecting inputs.
1913  */
1914  if ( ! nd_box_intersects(&nd_box, &(nd_stats->extent), ndims_max) )
1915  {
1916  POSTGIS_DEBUG(3, " search box does not overlap histogram, returning 0");
1917  return 0.0;
1918  }
1919 
1920  /* Search box completely contains histogram extent! */
1921  if ( nd_box_contains(&nd_box, &(nd_stats->extent), ndims_max) )
1922  {
1923  POSTGIS_DEBUG(3, " search box contains histogram, returning 1");
1924  return 1.0;
1925  }
1926 
1927  /* Calculate the overlap of the box on the histogram */
1928  if ( ! nd_box_overlap(nd_stats, &nd_box, &nd_ibox) )
1929  {
1930  POSTGIS_DEBUG(3, " search box overlap with stats histogram failed");
1931  return FALLBACK_ND_SEL;
1932  }
1933 
1934  /* Work out some measurements of the histogram */
1935  for ( d = 0; d < nd_stats->ndims; d++ )
1936  {
1937  /* Cell size in each dim */
1938  min[d] = nd_stats->extent.min[d];
1939  max[d] = nd_stats->extent.max[d];
1940  cell_size[d] = (max[d] - min[d]) / nd_stats->size[d];
1941  POSTGIS_DEBUGF(3, " cell_size[%d] : %.9g", d, cell_size[d]);
1942 
1943  /* Initialize the counter */
1944  at[d] = nd_ibox.min[d];
1945  }
1946 
1947  /* Move through all the overlap values and sum them */
1948  do
1949  {
1950  float cell_count, ratio;
1951  ND_BOX nd_cell;
1952 
1953  /* We have to pro-rate partially overlapped cells. */
1954  for ( d = 0; d < nd_stats->ndims; d++ )
1955  {
1956  nd_cell.min[d] = min[d] + (at[d]+0) * cell_size[d];
1957  nd_cell.max[d] = min[d] + (at[d]+1) * cell_size[d];
1958  }
1959 
1960  ratio = nd_box_ratio(&nd_box, &nd_cell, nd_stats->ndims);
1961  cell_count = nd_stats->value[nd_stats_value_index(nd_stats, at)];
1962 
1963  /* Add the pro-rated count for this cell to the overall total */
1964  total_count += cell_count * ratio;
1965  POSTGIS_DEBUGF(4, " cell (%d,%d), cell value %.6f, ratio %.6f", at[0], at[1], cell_count, ratio);
1966  }
1967  while ( nd_increment(&nd_ibox, nd_stats->ndims, at) );
1968 
1969  /* Scale by the number of features in our histogram to get the proportion */
1970  selectivity = total_count / nd_stats->histogram_features;
1971 
1972  POSTGIS_DEBUGF(3, " nd_stats->histogram_features = %f", nd_stats->histogram_features);
1973  POSTGIS_DEBUGF(3, " nd_stats->histogram_cells = %f", nd_stats->histogram_cells);
1974  POSTGIS_DEBUGF(3, " sum(overlapped histogram cells) = %f", total_count);
1975  POSTGIS_DEBUGF(3, " selectivity = %f", selectivity);
1976 
1977  /* Prevent rounding overflows */
1978  if (selectivity > 1.0) selectivity = 1.0;
1979  else if (selectivity < 0.0) selectivity = 0.0;
1980 
1981  return selectivity;
1982 }
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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