## Name

ST_LineSubstring — 返回两个小数位置之间的直线部分。

## Synopsis

`geometry ST_LineSubstring(`geometry a_linestring, float8 startfraction, float8 endfraction`)`;

`geography ST_LineSubstring(`geography a_linestring, float8 startfraction, float8 endfraction`)`;

## 描述

 这仅适用于 LINESTRING。 要在连续的 MULTILINESTRING 上使用，首先使用 ST_LineMerge将它们连接起来。
 从 1.1.1 版开始，此函数对 M 和 Z 值进行插值。 之前的版本将 Z 和 M 设置为未指定的值。

## 示例

```SELECT ST_AsText(ST_LineSubstring( 'LINESTRING (20 180, 50 20, 90 80, 120 40, 180 150)', 0.333, 0.666));
------------------------------------------------------------------------------------------------
LINESTRING (45.17311810399485 45.74337011202746, 50 20, 90 80, 112.97593050157862 49.36542599789519)
```

```SELECT ST_AsText(ST_LineSubstring( 'LINESTRING(25 50, 100 125, 150 190)', 0.333, 0.333));
------------------------------------------
POINT(69.2846934853974 94.2846934853974)
```

```
WITH data(id, geom) AS (VALUES
( 'A', 'LINESTRING( 0 0, 200 0)'::geometry ),
( 'B', 'LINESTRING( 0 100, 350 100)'::geometry ),
( 'C', 'LINESTRING( 0 200, 50 200)'::geometry )
)
SELECT id, i,
ST_AsText( ST_LineSubstring( geom, startfrac, LEAST( endfrac, 1 )) ) AS geom
FROM (
SELECT id, geom, ST_Length(geom) len, 100 sublen FROM data
) AS d
CROSS JOIN LATERAL (
SELECT i, (sublen * i) / len AS startfrac,
(sublen * (i+1)) / len AS endfrac
FROM generate_series(0, floor( len / sublen )::integer ) AS t(i)
-- skip last i if line length is exact multiple of sublen
WHERE (sublen * i) / len <
> 1.0
) AS d2;

id | i |            geom
----+---+-----------------------------
A  | 0 | LINESTRING(0 0,100 0)
A  | 1 | LINESTRING(100 0,200 0)
B  | 0 | LINESTRING(0 100,100 100)
B  | 1 | LINESTRING(100 100,200 100)
B  | 2 | LINESTRING(200 100,300 100)
B  | 3 | LINESTRING(300 100,350 100)
C  | 0 | LINESTRING(0 200,50 200)

```

```SELECT ST_AsText(ST_LineSubstring( 'LINESTRING(-118.2436 34.0522, -71.0570 42.3611)'::geography, 0.333, 0.666),6) AS geog_sub
, ST_AsText(ST_LineSubstring('LINESTRING(-118.2436 34.0522, -71.0570 42.3611)'::geometry, 0.333, 0.666),6) AS geom_sub;
---------------------------------------------------------------
geog_sub | LINESTRING(-104.167064 38.854691,-87.674646 41.849854)
geom_sub | LINESTRING(-102.530462 36.819064,-86.817324 39.585927)
```