Rebuild an index while it is being referenced in Oracle
Index rebuild operation is DDL and thus may lock the underlying table preventing you from updating it. However queries by nature do not lock (unless FOR UPDATE clause is specified) and can read from objects even if they're locked, thus, generally, no problems will arise during the index rebuild while queries are running (maybe except for some performance degradation during rebuild).

If you want to be able to update the table having the index which is being rebuilt, you can specify ONLINE clause:

SQL&gt; alter index table1_pk_idx rebuild online;


Otherwise the session performing DML will wait until the index is rebuilt.

For more info on index rebuild, ONLINE clause, and index rebuild restrictions, read the section "Rebuilding an Existing Index" in Administrator's Guide, and the section "ALTER INDEX" in SQL Language Reference.