Investigate/fix problems with duplicate index on mdl_block_grade_me.id column

[logan-reynolds]

Per comment from Moodle.org user Matteo Ricci, the following issue needs to be investigated and fixed to resolve problems with potential duplicate index on the column mdl_block_grade_me.id. Noting issue was originally happening on Oracle DB, and will need to be tested there to accurately reproduce:

Hi,
we have tried to install grade me on Moodle 3.0 version
with ORACLE DB and during the installation we have received this error:

ORA-01408: such column list already indexed
CREATE INDEX q_blocgradme_id_ix ON q_block_grade_me (id)
Error code: ddlexecuteerror

For the same type of error (ORA-01408) we have found in the web site
http://www.techonthenet.com/oracle/errors/ora01408.php ,
This explanation:
You tried to create an index on a set of columns in a table, but you've already indexed this set of columns.

If we comment this line in install.xml
KEY NAME="id" TYPE="foreign" FIELDS="id" REFTABLE="grade_items" REFFIELDS="id" PREVIOUS="primary" NEXT="courseid"
the installation goes ahead.

Using Mysql installation in phpmyadmin we have found the following warning:
The indexes PRIMARY and mdl_blocgradme_id_ix seem to be equal and one of them could possibly be removed.`