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8.8. 几何类型

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几何数据类型表示二维的空间物体。表 8.20展示了PostgreSQL中可以用的几何类型。

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表 8.20. 几何类型

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\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
名字存储大小说明表示方式
point16 字节平面上的点(x,y)
line24 字节无限直线{A,B,C}
lseg32 字节有限线段[(x1,y1),(x2,y2)]
box32 字节矩形框(x1,y1),(x2,y2)
path16+16n 字节闭合路径（类似多边形）((x1,y1),...)
path16+16n 字节开放路径[(x1,y1),...]
polygon40+16n字节多边形（类似于封闭路径）((x1,y1),...)
circle24 字节圆<(x,y),r>（圆心和半径）
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在所有这些类型中，单个坐标都以 double precision（float8）数值存储。

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我们提供了丰富的函数和操作符来进行各种几何操作，例如缩放、平移、旋转以及求交等，详见第 9.11 节。

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8.8.1. 点

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点是几何类型的基本二维构造块。用下面任一种语法描述point类型的值：

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( x , y )\n  x , y\n
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其中x和y分别是坐标，都是浮点数。

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点使用第一种语法输出。

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8.8.2. 直线

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直线由线性方程Ax + By + C = 0 表示，其中A和B不能同时为零。类型line 的值采用以下形式输入和输出：

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{ A, B, C }\n
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另外，还可以用下列任一形式输入：

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[ ( x1 , y1 ) , ( x2 , y2 ) ]\n( ( x1 , y1 ) , ( x2 , y2 ) )\n  ( x1 , y1 ) , ( x2 , y2 )\n    x1 , y1   ,   x2 , y2\n
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其中 (x1,y1) 和 (x2,y2) 是直线上不同的两点。

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8.8.3. 线段

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线段用其两个端点来表示。lseg类型的值用下面任一种语法指定：

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[ ( x1 , y1 ) , ( x2 , y2 ) ]\n( ( x1 , y1 ) , ( x2 , y2 ) )\n  ( x1 , y1 ) , ( x2 , y2 )\n    x1 , y1   ,   x2 , y2\n
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其中(x1,y1) 和 (x2,y2) 是线段的端点。

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线段使用第一种语法输出。

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8.8.4. 方框

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方框用其对角的点对表示。box类型的值使用下面任一种语法指定：

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( ( x1 , y1 ) , ( x2 , y2 ) )\n  ( x1 , y1 ) , ( x2 , y2 )\n    x1 , y1   ,   x2 , y2\n
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其中(x1,y1) 和 (x2,y2) 是方框的对角点。

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方框使用第二种语法输出。

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在输入时可以提供任意两个对角，但是值将根据需要被重新排列，并依次存储为右上角和左下角。

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8.8.5. 路径

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路径由一系列连接的点组成。路径可能是开放的，也就是认为列表中第一个点和最后一个点没有被连接起来；也可能是封闭的，这时认为第一个和最后一个点被连接起来。

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path类型的值用下面任一种语法指定：

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[ ( x1 , y1 ) , ... , ( xn , yn ) ]\n( ( x1 , y1 ) , ... , ( xn , yn ) )\n  ( x1 , y1 ) , ... , ( xn , yn )\n  ( x1 , y1   , ... ,   xn , yn )\n    x1 , y1   , ... ,   xn , yn\n
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其中的点是组成路径的线段的端点。方括弧（[]）表示一个开放的路径，圆括弧（()）表示一个封闭的路径。如第三种到第五种语法所示，当最外面的圆括号被省略时，路径将被假定为封闭。

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路径的输出视情况使用第一种或第二种语法。

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8.8.6. 多边形

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多边形由点的列表来表示，这些点就是多边形的顶点。多边形与封闭路径非常相似；其本质语义差异在于，多边形被认为包含其内部区域，而路径则不包含。

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多边形与路径在实现上的一个重要区别是，多边形的存储表示中包含其最小包围框。这会加快某些搜索操作，尽管在构造新多边形时，计算该包围框会带来额外开销。

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polygon类型的值用下列任一种语法指定：

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( ( x1 , y1 ) , ... , ( xn , yn ) )\n  ( x1 , y1 ) , ... , ( xn , yn )\n  ( x1 , y1   , ... ,   xn , yn )\n    x1 , y1   , ... ,   xn , yn\n
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其中的点是组成多边形边界的线段的端点。

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多边形的输出使用第一种语法。

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8.8.7. 圆

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圆由一个圆心和一个半径代表。circle类型的值用下面任一种语法指定：

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< ( x , y ) , r >\n( ( x , y ) , r )\n  ( x , y ) , r\n    x , y   , r\n
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其中(x,y)是圆心，而r是圆的半径。

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圆的输出用第一种语法。

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