PostGIS  2.5.0dev-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 1910 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().

1911 {
1912  int d; /* counter */
1913  float8 selectivity;
1914  ND_BOX nd_box;
1915  ND_IBOX nd_ibox;
1916  int at[ND_DIMS];
1917  double cell_size[ND_DIMS];
1918  double min[ND_DIMS];
1919  double max[ND_DIMS];
1920  double total_count = 0.0;
1921  int ndims_max;
1922 
1923  /* Calculate the overlap of the box on the histogram */
1924  if ( ! nd_stats )
1925  {
1926  elog(NOTICE, " estimate_selectivity called with null input");
1927  return FALLBACK_ND_SEL;
1928  }
1929 
1930  ndims_max = Max(nd_stats->ndims, gbox_ndims(box));
1931 
1932  /* Initialize nd_box. */
1933  nd_box_from_gbox(box, &nd_box);
1934 
1935  /*
1936  * To return 2D stats on an ND sample, we need to make the
1937  * 2D box cover the full range of the other dimensions in the
1938  * histogram.
1939  */
1940  POSTGIS_DEBUGF(3, " mode: %d", mode);
1941  if ( mode == 2 )
1942  {
1943  POSTGIS_DEBUG(3, " in 2d mode, stripping the computation down to 2d");
1944  ndims_max = 2;
1945  }
1946 
1947  POSTGIS_DEBUGF(3, " nd_stats->extent: %s", nd_box_to_json(&(nd_stats->extent), nd_stats->ndims));
1948  POSTGIS_DEBUGF(3, " nd_box: %s", nd_box_to_json(&(nd_box), gbox_ndims(box)));
1949 
1950  /*
1951  * Search box completely misses histogram extent?
1952  * We have to intersect in all N dimensions or else we have
1953  * zero interaction under the &&& operator. It's important
1954  * to short circuit in this case, as some of the tests below
1955  * will return junk results when run on non-intersecting inputs.
1956  */
1957  if ( ! nd_box_intersects(&nd_box, &(nd_stats->extent), ndims_max) )
1958  {
1959  POSTGIS_DEBUG(3, " search box does not overlap histogram, returning 0");
1960  return 0.0;
1961  }
1962 
1963  /* Search box completely contains histogram extent! */
1964  if ( nd_box_contains(&nd_box, &(nd_stats->extent), ndims_max) )
1965  {
1966  POSTGIS_DEBUG(3, " search box contains histogram, returning 1");
1967  return 1.0;
1968  }
1969 
1970  /* Calculate the overlap of the box on the histogram */
1971  if ( ! nd_box_overlap(nd_stats, &nd_box, &nd_ibox) )
1972  {
1973  POSTGIS_DEBUG(3, " search box overlap with stats histogram failed");
1974  return FALLBACK_ND_SEL;
1975  }
1976 
1977  /* Work out some measurements of the histogram */
1978  for ( d = 0; d < nd_stats->ndims; d++ )
1979  {
1980  /* Cell size in each dim */
1981  min[d] = nd_stats->extent.min[d];
1982  max[d] = nd_stats->extent.max[d];
1983  cell_size[d] = (max[d] - min[d]) / nd_stats->size[d];
1984  POSTGIS_DEBUGF(3, " cell_size[%d] : %.9g", d, cell_size[d]);
1985 
1986  /* Initialize the counter */
1987  at[d] = nd_ibox.min[d];
1988  }
1989 
1990  /* Move through all the overlap values and sum them */
1991  do
1992  {
1993  float cell_count, ratio;
1994  ND_BOX nd_cell = { {0.0, 0.0, 0.0, 0.0}, {0.0, 0.0, 0.0, 0.0} };
1995 
1996  /* We have to pro-rate partially overlapped cells. */
1997  for ( d = 0; d < nd_stats->ndims; d++ )
1998  {
1999  nd_cell.min[d] = min[d] + (at[d]+0) * cell_size[d];
2000  nd_cell.max[d] = min[d] + (at[d]+1) * cell_size[d];
2001  }
2002 
2003  ratio = nd_box_ratio(&nd_box, &nd_cell, nd_stats->ndims);
2004  cell_count = nd_stats->value[nd_stats_value_index(nd_stats, at)];
2005 
2006  /* Add the pro-rated count for this cell to the overall total */
2007  total_count += cell_count * ratio;
2008  POSTGIS_DEBUGF(4, " cell (%d,%d), cell value %.6f, ratio %.6f", at[0], at[1], cell_count, ratio);
2009  }
2010  while ( nd_increment(&nd_ibox, nd_stats->ndims, at) );
2011 
2012  /* Scale by the number of features in our histogram to get the proportion */
2013  selectivity = total_count / nd_stats->histogram_features;
2014 
2015  POSTGIS_DEBUGF(3, " nd_stats->histogram_features = %f", nd_stats->histogram_features);
2016  POSTGIS_DEBUGF(3, " nd_stats->histogram_cells = %f", nd_stats->histogram_cells);
2017  POSTGIS_DEBUGF(3, " sum(overlapped histogram cells) = %f", total_count);
2018  POSTGIS_DEBUGF(3, " selectivity = %f", selectivity);
2019 
2020  /* Prevent rounding overflows */
2021  if (selectivity > 1.0) selectivity = 1.0;
2022  else if (selectivity < 0.0) selectivity = 0.0;
2023 
2024  return selectivity;
2025 }
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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