what coalesce function you mean?
The mySQL DB has one, Oracle has (as PL/SQL function) one but MaxDB does provide such sql function.
Perhabs you want to use the VALUE() function for that ?
<a href="http://dev.mysql.com/doc/maxdb/en/cf/633171c03511d2a97100a0c9449261/content.htm">MaxDB Docu</a>
If you refer to the ALTER INDEX ... COALESCE command on oracle: that one neither makes sense in MaxDB nor is it available.
Concerning performance or stability there is no drawback in using the ORACLE mode.
SAP Netweaver acually uses it (or better it uses a copy of that mode).