当你的地理库中存储了太多信息，并且想提升查询效率，以应对高频访问场景时，应该如何操作呢？
答：构建空间索引
第一步：构建测试数据（已具备数据，则忽略此步骤）
测试数据使用反余弦，构建相对均衡，随机分布的点。
DROP TABLE IF EXISTS example1
;
CREATE TABLE example1 (
idcol serial NOT NULL,
geomcol geometry NULL,
CONSTRAINT example1_pk PRIMARY KEY (idcol),
CONSTRAINT enforce_srid CHECK (st_srid(geomcol) = 4326)
);
INSERT INTO example1(geomcol)
SELECT ST_SetSRID(
ST_MakePoint(
(random()*360.0) - 180.0,
(acos(1.0 - 2.0 * random()) * 2.0 - pi()) * 90.0 / pi()),
4326) as geomcol
FROM generate_series(1, 1000000) vtab;
第二步：在构建索引前，测试查询效率。
EXPLAIN ANALYZE VERBOSE
SELECT count(*)
FROM example1
WHERE ST_DWithin(geomcol::geography,ST_SetSRID(ST_MakePoint(6.9333,46.8167),4326)::geography,30 * 1609.34)
;
第三步：构建索引后，再次测试查询效率
CREATE INDEX example1_gpx ON example1 USING GIST (geography(geomcol));
EXPLAIN ANALYZE VERBOSE
SELECT count(*)
FROM example1
WHERE ST_DWithin(geomcol::geography,ST_SetSRID(ST_MakePoint(6.9333,46.8167),4326)::geography,30 * 1609.34)
;