Zabbix uses DBbegin_multiple_update(), DBexecute_overflowed_sql() and DBend_multiple_update() functions to compile multiple SQL queries into single query. On Oracle using separate queries might be better because of query caching.
Investigate performance difference on Oracle and if there are noticeable gains then change DBexecute_overflowed_sql() to immediately execute the query instead of buffering it.
Also review code where DBexecute_overflowed_sql() function is used. It's quite possible that in some places it's used after buffering mutiple queries - then the code must be changed to call it after each assembled query.