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Whats going on: I am working with postGIS and spatial datas. I am having trouble to fully understand the R-tree algorithm.

Example query: In which town is point P located?

My understanding so far is: Each polygon has a boundingBox, this boundingBox has an idex. These boundingBoxes are then merged into another larger boundingBox which also has an idex. Continuing the process creates the R-tree. With the help of the R-tree the query is much faster. But ...

My question is at what point are these indexes created? Does the R-tree exists per se? Or is the R-tree created on the run time when I start a query?

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You have to create a spatial index.

Create index idx_mytable_geom ON my_table USING gist(geom);

Then it is often a good idea to run

ANALYZE mytable;

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