10.3.3 SPATIAL Index Optimization

MySQL permits creation of SPATIAL indexes on NOT NULL geometry-valued columns (see Section 13.4.10, “Creating Spatial Indexes”). The optimizer checks the SRID attribute for indexed columns to determine which spatial reference system (SRS) to use for comparisons, and uses calculations appropriate to the SRS. (Prior to MySQL 8.4, the optimizer performs comparisons of SPATIAL index values using Cartesian calculations; the results of such operations are undefined if the column contains values with non-Cartesian SRIDs.)

For comparisons to work properly, each column in a SPATIAL index must be SRID-restricted. That is, the column definition must include an explicit SRID attribute, and all column values must have the same SRID.

The optimizer considers SPATIAL indexes only for SRID-restricted columns:

Indexes on columns restricted to a Cartesian SRID enable Cartesian bounding box computations.
Indexes on columns restricted to a geographic SRID enable geographic bounding box computations.

The optimizer ignores SPATIAL indexes on columns that have no SRID attribute (and thus are not SRID-restricted). MySQL still maintains such indexes, as follows:

They are updated for table modifications (INSERT, UPDATE, DELETE, and so forth). Updates occur as though the index was Cartesian, even though the column might contain a mix of Cartesian and geographical values.
They exist only for backward compatibility (for example, the ability to perform a dump in MySQL 8.2 and restore in MySQL 8.3). Because SPATIAL indexes on columns that are not SRID-restricted are of no use to the optimizer, each such column should be modified:
- Verify that all values within the column have the same SRID. To determine the SRIDs contained in a geometry column col_name, use the following query:
```
SELECT DISTINCT ST_SRID(col_name) FROM tbl_name;
```
  If the query returns more than one row, the column contains a mix of SRIDs. In that case, modify its contents so all values have the same SRID.
- Redefine the column to have an explicit SRID attribute.
- Recreate the SPATIAL index.