Session14

Session14 - CSCI585 CSCI585 Spatial Index Structures...

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Unformatted text preview: CSCI585 CSCI585 Spatial Index Structures Instructor: Cyrus Shahabi CSCI585 CSCI585 Outline Grid File Z-ordering Hilbert Curve Quad Tree PM PR R Tree (next session) R* Tree R+ Tree CSCI585 CSCI585 Grid File Hashing methods for multidimensional points (extension of Extensible hashing) Idea: Use a grid to partition the space b each cell is associated with one page Two disk access principle (exact match) CSCI585 CSCI585 Grid File Start with one bucket for the whole space. Select dividers along each dimension. Partition space into cells Dividers cut all the way. CSCI585 CSCI585 Grid File Each cell corresponds to 1 disk page. Many cells can point to the same page. Cell directory potentially exponential in the number of dimensions CSCI585 CSCI585 Grid File Implementation Dynamic structure using a grid directory Grid array: a 2 dimensional array with pointers to buckets (this array can be large, disk resident) G(0,, nx-1, 0, , ny-1) Linear scales: Two 1 dimensional arrays that used to access the grid array (main memory) X(0, , nx-1), Y(0, , ny-1) CSCI585 CSCI585 Example Linear scale X Linear scale Y Grid Directory Buckets/Disk Blocks CSCI585 CSCI585 The scales do not need to be uniform X 1 X 2 X 3 X 4 Y 1 Y 2 Y 3 Scales Grid-file buckets CSCI585 CSCI585 Grid File Search Exact Match Search: at most 2 I/Os assuming linear scales fit in memory. First use liner scales to determine the index into the cell directory access the cell directory to retrieve the bucket address (may cause 1 I/O if cell directory does not fit in memory) access the appropriate bucket (1 I/O) Range Queries: use linear scales to determine the index into the cell directory.directory....
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Session14 - CSCI585 CSCI585 Spatial Index Structures...

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