# Filtering using geohash covers

You can use geohash covers to filter objects for spatial processing, thereby greatly improving query performance.

For example, you can use geohash covers to quickly determine whether it is possible that two
geometries intersect:

To filter geometries using geohash covers:

- Identify the base tables involved in the spatial relationship function. For example, the base
tables involved in the following st_intersects function are SPX.TABLE2011 and
SPX.TABLE2019:
`SELECT a.*, b.TERR_FCS from SPX.TABLE2011 as a, SPX.TABLE2019 as b WHERE a.SST_DC = 'CA' and a.DC_ZIP = '95136' and st_intersects(a.SHAPE,b.SHAPE);`

- Note the primary key and geometry column of each of the base tablesthat are involved in the spatial relationship function. For example:
- In SPX.TABLE2011,
`a.shape2011_key`

and`a.shape`

- In SPX.TABLE2019,
`b.shape2019_key`

and`b.shape`

- In SPX.TABLE2011,
- Determine the size of the geometries to be covered, and use this information to decide which depth to use. The geohash covers of two geometries that are to be compared must be computed using the same depth. An attempt to compare geohash values computed using different depths is invalid. The following examples use a depth of 23.
- Create a geohash-filter table for each base table, and use the st_geohashcover function to populate each geohash-filter table with the geohash values that
correspond to the geohash covers of the geometries. For
example:
`Create table SPX.FILTERTABLE2011 (shape2011_key integer, geohash BIGINT) INSERT into SPX.FILTERTABLE2011 (select g.shape2011_key, t.geohash from SPX.TABLE2011 as g, table(st_geohashcover_medium(g.shape)) as t) Create table SPX.FILTERTABLE2019 (shape2019_key integer, geohash BIGINT) INSERT into SPX.FILTERTABLE2019 (select g.shape2019_key, t.geohash from SPX.TABLE2019 as g, table(st_geohashcover_medium(g.shape)) as t)`

- Rewrite the original query to use the filter tables to reduce the amount of data that needs to
be compared. For
example:
`SELECT a.*, b.TERR_FCS from SPX.TABLE2011 a, SPX.TABLE2019 b,`

**SPX.FILTERTABLE2011 as c, SPX.FILTERTABLE2019 as d**WHERE a.SST_DC = 'CA’ and a.DC_ZIP = '95136’ and**c.shape2011_key = a.shape2011_key and d.shape2019_key = b.shape2019_key and c.geohash = d.geohash and**st_intersects(a.SHAPE,b.SHAPE);