Functions that use indexes to optimize queries
A specialized group
of spatial functions, called comparison functions, can
improve query performance by exploiting spatial grid indexes. Each
of these functions compares two geometries with one another.
If
the results of the comparison meet certain criteria, the function
returns a value of 1; if the results fail to meet the criteria, the
function returns a value of 0. If the comparison cannot be performed,
the function can return a null value.
For example, the function ST_Overlaps compares two geometries that have the same dimension (for example, two linestrings or two polygons). If the geometries overlap partway, and if the space covered by the overlap has the same dimension as the geometries, ST_Overlaps returns a value of 1.
Table 1 shows which comparison functions
can use a spatial grid index:
Comparison function  Can use spatial grid index 

EnvelopesIntersect  Yes 
ST_Contains  Yes 
ST_Crosses  Yes 
ST_Distance  Yes 
ST_EnvIntersects  Yes 
ST_Equals  Yes 
ST_Intersects  Yes 
ST_MBRIntersects  Yes 
ST_Overlaps  Yes 
ST_Touches  Yes 
ST_Within  Yes 
Because of the time and memory required to execute
a function, such execution can involve considerable processing. Furthermore,
the more complex the geometries that are being compared, the more
complex and timeintensive the comparison will be. The specialized
functions listed previously can complete their operations more quickly
if they can use a spatial index to locate geometries. To enable such
a function to use a spatial index, observe all of the following rules:
 The function must be specified in a WHERE clause. If it is specified in a SELECT, HAVING, or GROUP BY clause, a spatial index cannot be used.
 The function must be the expression on left of the predicate.
 The operator that is used in the predicate that compares the result of the function with another expression must be an equal sign, with one exception: the ST_Distance function must use the less than operator.
 The expression on the right of the predicate must be the constant 1, except when ST_Distance is the function on the left.
 The operation must involve a search in a spatial column on which a spatial index is defined.
For example:
SELECT c.name, c.address, c.phone
FROM customers AS c, bank_branches AS b
WHERE db2gse.ST_Distance(c.location, b.location) < 10000
and b.branch_id = 3
Table 2 shows correct
and incorrect ways of creating spatial queries to utilize a spatial
index.
Queries that reference spatial functions  Rules violated 


No condition is violated in this example. 

The spatial function ST_Length does not compare geometries and cannot utilize a spatial index. 

The function must be an expression on the left side of the predicate. 

Equality comparisons must use the integer constant 1. 

No spatial index exists on either of the arguments for the function, so no index can be utilized. 