MDL-66253 made it so the raw_event_retrieval_strategy can end up making this part of it's query to fetch events:

SELECT ev.modulename,

       ev.instance,

       ev.eventtype,

       MIN(ev.priority) as priority

FROM ($subqueryunion) ev

GROUP BY ev.modulename, ev.instance, ev.eventtype

The problem is that it made it so $subqueryunion can just be "{event}", resulting in "FROM (mdl_event) ev". Postgres doesn't accept a table name alone as a valid subquery within the parenthesis and fails.

This is fixed by removing the parenthesis around the table name when that's the only thing in the subqueryunion.

See: https://www.postgresql.org/docs/9.5/sql-select.html