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Change feature density computation algorithm

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Now parameterize by the number of levels to traverse and
start with the level that covers the extent of the table
with at least N*N tiles (N=4).
This commit is contained in:
Javier Goizueta 2015-12-16 16:39:38 +01:00
parent 554464e43e
commit f7857945c1
1 changed files with 38 additions and 15 deletions
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53
scripts-available/CDB_Overviews.sql
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@ -1,10 +1,11 @@
CREATE OR REPLACE FUNCTION _CDB_Feature_Density(reloid REGCLASS, max_z integer)
CREATE OR REPLACE FUNCTION _CDB_Feature_Density(reloid REGCLASS, nz integer)
RETURNS FLOAT8
AS $$
DECLARE
fd FLOAT8;
min_features TEXT;
n integer = 4;
c FLOAT8;
BEGIN
-- TODO: for small total count or extents we could just:
-- EXECUTE 'SELECT Count(*)/ST_Area(ST_Extent(the_geom_webmercator)) FROM ' || reloid::text || ';' INTO fd;
@ -16,26 +17,48 @@ AS $$
-- the area of tiles at level Z: c*c*power(2, -2*z)
-- with c = CDB_XYZ_Resolution(-8) (earth circumference)
min_features = '500';
SELECT CDB_XYZ_Resolution(-8) INTO c;
-- TODO: compute min_z and seed tiles based on reloid extents
-- so as to have at least N^2 tiles that cover the extents for some N
-- For the time being we use the root tile as the seed.
-- We first compute a set of *seed* tiles, of the minimum Z level, z0, such that
-- they cover the extent of the table and we have at least n of them in each
-- linear dimension (i.e. at least n*n tiles cover the extent).
-- We compute the number of features in these tiles, and recursively in
-- subtiles up to level z0 + nz. Then we compute the maximum of the feature
-- density (per tile area in webmercator squared meters) for all the
-- considered tiles.
EXECUTE Format('
WITH RECURSIVE t(x, y, z, e) AS (
SELECT 0, 0, 0, (
SELECT count(*) FROM %1$s
WHERE the_geom_webmercator && CDB_XYZ_Extent(0, 0, 0)
WITH ext AS (SELECT ST_Extent(the_geom_webmercator) g FROM %1$s),
base AS (
SELECT (-floor(log(2, (greatest(ST_XMax(ext.g)-ST_XMin(ext.g), ST_YMax(ext.g)-ST_YMin(ext.g))/(%4$s*%5$s))::numeric)))::integer z
FROM ext
),
lim AS (
SELECT
FLOOR((ST_XMin(ext.g)+CDB_XYZ_Resolution(0)*128)/(CDB_XYZ_Resolution(base.z)*256))::integer x0,
FLOOR((ST_XMax(ext.g)+CDB_XYZ_Resolution(0)*128)/(CDB_XYZ_Resolution(base.z)*256))::integer x1,
FLOOR((CDB_XYZ_Resolution(0)*128-ST_YMin(ext.g))/(CDB_XYZ_Resolution(base.z)*256))::integer y1,
FLOOR((CDB_XYZ_Resolution(0)*128-ST_YMax(ext.g))/(CDB_XYZ_Resolution(base.z)*256))::integer y0
FROM ext, base
),
seed AS (
SELECT xt, yt, base.z, (
SELECT count(*) FROM %1$s
WHERE the_geom_webmercator && CDB_XYZ_Extent(xt, yt, base.z)
) e
FROM base, lim, generate_series(lim.x0, lim.x1) xt, generate_series(lim.y0, lim.y1) yt
)
SELECT * from seed
UNION ALL
SELECT x*2 + xx, y*2 + yy, z+1, (
SELECT x*2 + xx, y*2 + yy, t.z+1, (
SELECT count(*) FROM %1$s
WHERE the_geom_webmercator && CDB_XYZ_Extent(x*2 + xx, y*2 + yy, z+1)
WHERE the_geom_webmercator && CDB_XYZ_Extent(x*2 + xx, y*2 + yy, t.z+1)
)
FROM t, (VALUES (0, 0), (0, 1), (1, 1), (1, 0)) AS c(xx, yy)
WHERE e > %2$s AND z < %3$s
FROM t, base, (VALUES (0, 0), (0, 1), (1, 1), (1, 0)) AS c(xx, yy)
WHERE t.e > %2$s AND t.z < (base.z + %3$s)
)
SELECT MAX(e/ST_Area(CDB_XYZ_Extent(x,y,z))) FROM t where e > 0;
', reloid::text, min_features, max_z)
', reloid::text, min_features, nz, n, c)
INTO fd;
RETURN fd;
END
@ -46,13 +69,13 @@ RETURNS INTEGER
AS $$
DECLARE
lim FLOAT8 := 500; -- TODO: determine/parameterize this
max_z integer := 14;
nz integer := 4;
fd FLOAT8;
c FLOAT8;
BEGIN
-- Compute fd as an estimation of the (maximum) number
-- of features per unit of tile area (in webmercator squared meters)
SELECT _CDB_Feature_Density(reloid, max_z) INTO fd;
SELECT _CDB_Feature_Density(reloid, nz) INTO fd;
-- lim maximum number of (desiderable) features per tile
-- we have c = 2*Pi*R = CDB_XYZ_Resolution(-8) (earth circumference)
-- ta(z): tile area = power(c*power(2,z), 2) = c*c*power(2,2*z)