Can;t get MySql/Mariadb to use spatial index

This is using 5.5.68-MariaDB. Yes, I know this is very old but I have to use this version for this application for the time being.

The explain and the performance of the query clearly show the indexes aren't being used. Here's the info:

explain select bdcfabric.location_id from bdccoverage, bdcfabric where
 st_intersects(bdccoverage.shape,bdcfabric.bxlocation)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: bdccoverage
         type: ALL
possible_keys: SHAPE
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 886
        Extra:
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: bdcfabric
         type: ALL
possible_keys: bxlocation_idx
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1105588
        Extra: Using where; Using join buffer (flat, BNL join)

The indexes:

show indexes from bdccoverage\G
*************************** 4. row ***************************
        Table: bdccoverage
   Non_unique: 1
     Key_name: SHAPE
 Seq_in_index: 1
  Column_name: SHAPE
    Collation: A
  Cardinality: NULL
     Sub_part: 32
       Packed: NULL
         Null:
   Index_type: SPATIAL
      Comment:
Index_comment:

show indexes from bdcfabric\G
*************************** 2. row ***************************
        Table: bdcfabric
   Non_unique: 1
     Key_name: bxlocation_idx
 Seq_in_index: 1
  Column_name: bxlocation
    Collation: A
  Cardinality: NULL
     Sub_part: 32
       Packed: NULL
         Null:
   Index_type: SPATIAL
      Comment:
Index_comment:

The schema:

+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| SHAPE      | geometry    | NO   | MUL |         |                |

describe bdcfabric;
+-------------------------+------------+------+-----+---------+-------+
| Field                   | Type       | Null | Key | Default | Extra |
+-------------------------+------------+------+-----+---------+-------+
| bxlocation              | point      | NO   | MUL |         |       |
+-------------------------+------------+------+-----+---------+-------+

Why aren't the indexes being used?

Your query needs to run all locations of bdcfabric.bxlocation against bdccoverage.shape in the other table. These kinds of joins where the join criteria is complex (like st_intersects) don't lead themselves to index usage.

Even on this old version, increasing join_buffer_size / join_buffer_space_limit. Look at the session rows_read on the default size for the query compared to when you increase it.