ST_LeafN

语法

trajectory ST_LeafN(trajectory traj, integer pos);

参数

返回值

返回一个单点的轨迹对象。

描述

获取第n个轨迹点。轨迹点内包含空间、时间和属性信息。

示例

With traj_table AS
       (
         SELECT ST_MakeTrajectory('STPOINT'::leaftype, st_geomfromtext('LINESTRING (114 35, 114 35, 116 37)', 4326), ARRAY['2010-1-11 14:30'::timestamp,'2010-1-11 14:30', '2010-1-11 15:00'], '{"leafcount":3,"attributes":{"velocity": {"type": "integer", "length": 2,"nullable" : true,"value": [120, 130, 140]}, "accuracy": {"type": "float", "length": 4, "nullable" : false,"value": [120, 130, 140]}, "bearing": {"type": "float", "length": 8, "nullable" : false,"value": [120, 130, 140]}, "acceleration": {"type": "string", "length": 20, "nullable" : true,"value": ["120", "130", "140"]}, "active": {"type": "timestamp", "nullable" : false,"value": ["Fri Jan 01 14:30:00 2010", "Fri Jan 01 15:00:00 2010", "Fri Jan 01 15:30:00 2010"]}}, "events": [{"1" : "Fri Jan 01 14:30:00 2010"}, {"2" : "Fri Jan 01 15:00:00 2010"}, {"3" : "Fri Jan 01 15:30:00 2010"}]}') traj
       )
SELECT ST_LeafN(traj, 1) FROM traj_table;

返回结果如下：

{"trajectory":{"version":1,"type":"STPOINT","leafcount":1,"start_time":"2010-01-11 14:30:00","end_time":"2010-01-11 14:30:00","spatial":"SRID=4326;POINT(114 35)","timeline":["2010-01-11 14:30:00"],"attributes":{"leafcount":1,"velocity":{"type":"integer","length":2,"nullable":true,"value":[130]},"accuracy":{"type":"float","length":4,"nullable":false,"value":[130.0]},"bearing":{"type":"float","length":8,"nullable":false,"value":[130.0]},"acceleration":{"type":"string","length":20,"nullable":true,"value":["130"]},"active":{"type":"timestamp","length":8,"nullable":false,"value":["2010-01-01 15:00:00"]}},"events":[{"1":"2010-01-01 14:30:00"},{"2":"2010-01-01 15:00:00"},{"3":"2010-01-01 15:30:00"}]}}