RDBMS reindexing feature use queries based on LIMIT and OFFSET clauses which are not enabled by default. However, you can ensure they are enabled by executing the following

$ db2set DB2_COMPATIBILITY_VECTOR=MYS
$ db2stop
$ db2start

Statistics is collected automatically starting from DB2 Version 9, however it is needed to launch statistics collection manually during the very first start, otherwise it could be very long. You need to run 'RUNSTATS' command

RUNSTATS ON TABLE <scheme>.<table> WITH DISTRIBUTION AND INDEXES ALL

for JCR_SITEM (or JCR_MITEM) and JCR_SVALUE (or JCR_MVALUE) tables.