This topic describes TrajGiST indexing, which is an extension to GiST indexing. You can create a TrajGiST index on a column that stores trajectory data.
Background information
TrajGiST performs better than GiST in the following ways:
- TrajGiST provides a better method that is used to estimate the overheads of indexes. If more than one index is created, TrajGiST can better select an index than GiST.
- TrajGiST supports upward compatibility of indexes. If an accurate result cannot be obtained because the index does not provide sufficient information, you can still obtain a coarse-grained result by using an index-based scan. For example, you have created an index only on the t axis, but you want to call the ST_{2DT}Intersects function. In this case, you can filter out the trajectories that do not fall within a specified time range based on the index on the t axis.
Syntax
CREATE INDEX [index_name] on table_name USING TRAJGIST(traj_col [operator_family]);
- index_name: the name of the index. This parameter is optional.
- table_name: the name of the table to which the column belongs.
- traj_col: the name of the column.
- operator_family: the operator family that is used to create the index. Default value: trajgist_ops_3dt. This parameter is optional.
Note The created index accelerates queries that are run by operators and the following
functions: ST_ndIntersect, ST_ndDWithin, ST_ndContains, and ST_ndWithin.
Supported operator families
The following table describes the operator families that are supported by GiST.
Operator family | Description |
---|---|
trajgist_ops_z | Creates an index on the z axis. This type of index supports queries that cover only the z axis. |
trajgist_ops_t | Creates an index on the t axis. This type of index supports queries that cover only the t axis. |
trajgist_ops_2d | Creates an index on the x and y axes. This type of index supports queries that cover only the x and y axes. |
trajgist_ops_2dt | Creates an index on the x, y, and t axes. This type of index supports queries that cover the x, y, and t axes. |
trajgist_ops_3d | Creates an index on the x, y, and z axes. This type of index supports queries that cover the x, y, and z axes. |
trajgist_ops_3dt | Creates an index on the x, y, z, and t axes. This type of index supports all of the queries that are supported by the preceding five operator families. |
Note TrajGiST allows you to create an index on a single column. It does not allow you to
create an index on multiple columns including the columns that store other types of
data than trajectory data.