The problem occurs in 10.2 and earlier releases of MariaDB Server because the
Partition Engine was not pushing the engine conditions to the underlying
storage engine of each partition.  This caused Spider to return the first 5
rows in the table with the data provided by the customer.  2 of the 5 rows
did not qualify the WHERE clause, so they were removed from the result set by
the server.

To fix the problem, I have back-ported support for engine condition pushdown
in the Partition Engine from MariaDB Server 10.3 to 10.2 and 10.1.  In 10.3
and 10.4 I have merged the comments and the test case.

Author:
  Jacob Mathew.

Reviewer:
  Kentoku Shiba.

Merged:
  Commit eb2ca3d4 on branch bb-10.2-MDEV-16912