"Internal unindexed search" and "Partially Unindexed Filter" errors in Directory Server error logs

Issue

• In Directory Server error logs, plenty of errors as shown below about partial unindexed searches from syncrepl plugin even though there are indexes defined for the attributes specified in the filter.

ERR - log_result - Internal unindexed search: source (cn=server,cn=plugins,cn=config) search base="CN=xx,OU=xx,OU=xx,OU=xx,O=xx,C=xx" filter="objectclass=*" etime=0.000186693 nentries=1  notes=U details="Partially Unindexed Filter"
ERR - log_result - Internal unindexed search: source (cn=server,cn=plugins,cn=config) search base="CN=xx,OU=xx,OU=xx,OU=xx,O=xx,C=xx" filter="objectclass=*" etime=0.000186693 nentries=1  notes=U details="Partially Unindexed Filter"