Question

我有一个存储半径22km的ST_Buffer的表（SRID = 4326）。我正在尝试以下查询：

line = "ST_GeometryFromText('SRID=4326;LINESTRING(%s)')" % (line)
cur.execute("SELECT ST_AsText(ST_Difference(line,(SELECT ST_Union(buffers.the_geog::geometry)  
FROM buffers WHERE ST_Intersects( line ,buffers.the_geog::geometry )))) FROM %s as line"%(line))

EXPlAIN ANALYZE查询给出了以下结果：

[('Function Scan on line  (cost=0.00..2741841.15 rows=1 width=32) (actual time=10345.785..10345.789 rows=1 loops=1)',), 
('  SubPlan 1',), ('    ->  Aggregate  (cost=2741841.12..2741841.13 rows=1 width=576) (actual time=10345.434..10345.435 rows=1 loops=1)',), 
('          ->  Seq Scan on buffers  (cost=0.00..2741839.82 rows=261 width=576) (actual time=920.965..10051.716 rows=841 loops=1)',), 
('                Filter: ((line.line && (the_geog)::geometry) AND _st_intersects(line.line, (the_geog)::geometry))',), 
('                Rows Removed by Filter: 7814377',), ('Total runtime: 10346.496 ms',)]

正如您所看到的，ST_Intersects未使用GIST索引。如何强制查询使用GIST索引？

我的表结构是：

cur.execute("CREATE TABLE buffers(gid serial PRIMARY KEY, the_geog geography(POLYGON,4326) );")

索引是：

CREATE INDEX tindex ON buffer USING GIST ( the_geog );

Answer 1

假设您确实运行了分析，可能是因为子查询。在没有子查询的情况下编写查询时，它将使用空间索引。像这样：

select ST_AsText(ST_Difference(line.line, ST_Union(buffers.the_geom::geometry))) 
FROM line,buffers WHERE 
ST_Intersects( line.line ,buffers.the_geom::geometry )
group by line.line;

如何强制ST_Intersects使用空间索引而不是普通的btree索引？

1 个答案: