Recall that spatial index is one of the three key features of a spatial database. Indexes are what make using a spatial database for large data sets possible. Without indexing, any search for a feature would require a "sequential scan" of every record in the database. Indexing speeds up searching by organizing the data into a search tree which can be quickly traversed to find a particular record.
Spatial indices are one of the greatest assets of PostGIS. In the previous example building spatial joins requires comparing whole tables with each other. This can get very costly: joining two tables of 10,000 records each without indexes would require 100,000,000 comparisons; with indexes the cost could be as low as 20,000 comparisons.
When we loaded the
nyc_census_blocks table, the pgShapeLoader
automatically created a spatial index called
To demonstrate how important indexes are for performance, let's search
nyc_census_blocks without our spatial index.
Our first step is to remove the index.
DROP INDEX nyc_census_blocks_geom_idx;
DROP INDEXstatement drops an existing index from the database system. For more information, see the PostgreSQL documentation.
Now, watch the "Timing" meter at the lower right-hand corner of the pgAdmin query window and run the following. Our query searches through every single census block in order to identify the Broad Street entry.
SELECT blocks.blkid FROM nyc_census_blocks blocks JOIN nyc_subway_stations subways ON ST_Contains(blocks.geom, subways.geom) WHERE subways.name = 'Broad St';
blkid ----------------- 360610007001009
nyc_census_blocks table is very small (only a few thousand
records) so even without an index, the query only takes 55 ms on my
Now add the spatial index back in and run the query again.
CREATE INDEX nyc_census_blocks_geom_idx ON nyc_census_blocks USING GIST (geom);
USING GISTclause tells PostgreSQL to use the generic index structure (GIST) when building the index. If you receive an error that looks like
ERROR: index row requires 11340 bytes, maximum size is 8191when creating your index, you have likely neglected to add the
On my test computer the time drops to 9 ms. The larger your table, the larger the relative speed improvement of an indexed query will be.
How Spatial Indexes Work
Standard database indexes create a hierarchical tree based on the values of the column being indexed. Spatial indexes are a little different -- they are unable to index the geometric features themselves and instead index the bounding boxes of the features.
In the figure above, the number of lines that intersect the yellow star is one, the red line. But the bounding boxes of features that intersect the yellow box is two, the red and blue ones.
The way the database efficiently answers the question "what lines intersect the yellow star" is to first answer the question "what boxes intersect the yellow box" using the index (which is very fast) and then do an exact calculation of "what lines intersect the yellow star" only for those features returned by the first test.
For a large table, this "two pass" system of evaluating the approximate index first, then carrying out an exact test can radically reduce the amount of calculations necessary to answer a query.
Both PostGIS and Oracle Spatial share the same "R-Tree"[^1] spatial index structure. R-Trees break up data into rectangles, and sub-rectangles, and sub-sub rectangles, etc. It is a self-tuning index structure that automatically handles variable data density and object size.