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Spatial Indexing SAMs

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Spatial Indexing Point Access Methods can index only points. What about regions? Z-ordering and quadtrees Use the transformation technique and a PAM New methods: Spatial Access Methods SAMs R-tree and variations

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Problem Given a collection of geometric objects (points, lines, polygons,...) organize them on disk, to answer spatial queries (range, nn, etc)

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Transformation Technique Map an d-dim MBR into a point: ex. [(x min, x max ) (y min, y max )] => (x min, x max, y min, y max ) Use a PAM to index the 2d points Given a range query, map the query into the 2d space and use the PAM to answer it

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R-trees [Guttman 84] Main idea: allow parents to overlap! => guaranteed 50% utilization => easier insertion/split algorithms. (only deal with Minimum Bounding Rectangles - MBRs)

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R-trees A multi-way external memory tree Index nodes and data (leaf) nodes All leaf nodes appear on the same level Every node contains between m and M entries The root node has at least 2 entries (children)

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Example eg., w/ fanout 4: group nearby rectangles to parent MBRs; each group -> disk page A B C D E F G H J I

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Example F=4 A B C D E F G H I J P1 P2 P3 P4 FGDEHIJABC

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Example F=4 A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC

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R-trees - format of nodes {(MBR; obj_ptr)} for leaf nodes P1P2P3P4 ABC x-low; x-high y-low; y-high... obj ptr...

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R-trees - format of nodes {(MBR; node_ptr)} for non-leaf nodes P1P2P3P4 ABC x-low; x-high y-low; y-high... node ptr...

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R-trees:Search A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC

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R-trees:Search A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC

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R-trees:Search Main points: every parent node completely covers its ‘children’ a child MBR may be covered by more than one parent - it is stored under ONLY ONE of them. (ie., no need for dup. elim.) a point query may follow multiple branches. everything works for any(?) dimensionality

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R-trees:Insertion A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC X X Insert X

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R-trees:Insertion A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC Y Insert Y

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R-trees:Insertion Extend the parent MBR A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC Y Y

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R-trees:Insertion How to find the next node to insert the new object? Using ChooseLeaf: Find the entry that needs the least enlargement to include Y. Resolve ties using the area (smallest) Other methods (later)

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R-trees:Insertion If node is full then Split : ex. Insert w A B C D E F G H I J P1 P2 P3 P4 P1P2P3P4 FGDEHIJABC W K K

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R-trees:Insertion If node is full then Split : ex. Insert w A B C D E F G H I J P1 P2 P3 P4 Q1Q2FGDEHIJAB W K CKW P5 P1P5P2P3 P4 Q1 Q2

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R-trees:Split Split node P1: partition the MBRs into two groups. A B C W K P1 (A1: plane sweep, until 50% of rectangles) A2: ‘linear’ split A3: quadratic split A4: exponential split: 2 M-1 choices

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R-trees:Split pick two rectangles as ‘seeds’; assign each rectangle ‘R’ to the ‘closest’ ‘seed’ seed1 seed2 R

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R-trees:Split pick two rectangles as ‘seeds’; assign each rectangle ‘R’ to the ‘closest’ ‘seed’: ‘closest’: the smallest increase in area seed1 seed2 R

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R-trees:Split How to pick Seeds: Linear:Find the highest and lowest side in each dimension, normalize the separations, choose the pair with the greatest normalized separation Quadratic: For each pair E1 and E2, calculate the rectangle J=MBR(E1, E2) and d= J-E1-E2. Choose the pair with the largest d

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R-trees:Insertion Use the ChooseLeaf to find the leaf node to insert an entry E If leaf node is full, then Split, otherwise insert there Propagate the split upwards, if necessary Adjust parent nodes

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R-Trees:Deletion Find the leaf node that contains the entry E Remove E from this node If underflow: Eliminate the node by removing the node entries and the parent entry Reinsert the orphaned (other entries) into the tree using Insert Other method (later)

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R-trees: Variations R+-tree: DO not allow overlapping, so split the objects (similar to z-values) R*-tree: change the insertion, deletion algorithms (minimize not only area but also perimeter, forced re-insertion ) Hilbert R-tree: use the Hilbert values to insert objects into the tree

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