I got a question left... If i add the linestring-column to the ways table _after_ I imported all the data, is there a way to fill this column without re-import everything?
Cheers. 2011/12/22 Michel Seuthe <[email protected]> > Thanks for your detailed reply, Brett. Makes sense. > > > 2011/12/22 Brett Henderson <[email protected]> > >> Hi Michel, >> >> Yes, there is redundant data stored in the pgsnapshot schema. It's there >> for performance reasons. If you compare the pgsnapshot schema to the >> earlier pgsimple schema you'll notice that the way.nodes column is new, but >> that the way_nodes table has always existed. Both way->node mechanisms are >> used for different purposes. >> >> The pgsnapshot schema relies heavily on the PostgreSQL CLUSTER feature >> where table contents can be sorted to align with one of the indexes. The >> nodes and ways tables are both organised by geographical location which >> drastically improves performance when performing bounding box style queries >> because data in the same geographical area will be stored close together on >> disk. The way_nodes table can't make use of geographical clustering which >> means that joining the ways table to the nodes table via way_nodes is very >> slow for large amounts of data due to large amounts of disk seeks. >> Therefore the way->node information is duplicated inside the ways table for >> fast lookup of nodes within ways. This schema includes tags directly >> against the node, way and relation tables in hstore columns for exactly the >> same reason. On typical SATA disks, the pgsnapshot schema performs >> approximately 10 times faster than the pgsimple schema for bounding box >> queries. >> >> I can't drop the way_node table because it is still needed for two >> reasons (may be others I've forgotten). 1. It is used for replication when >> a node changes and the related way geometries have to be updated. 2. It is >> used for bounding box queries if the ways table doesn't have any geometric >> columns configured. >> >> I hope that makes it clearer. >> Brett >> >> >
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