LOB columns are subject to the following rules and restrictions:

• You cannot specify a LOB as a primary key column.
• You cannot specify LOB columns in the ORDER BY clause of a query, the GROUP BY clause of a query, or an aggregate function.
• You cannot specify a LOB column in a SELECT... DISTINCT or SELECT... UNIQUE statement or in a join. However, you can specify a LOB attribute of an object type column in a SELECT... DISTINCT statement, a query that uses the UNION, or a MINUS set operator if the object type of the column has a MAP or ORDER function defined on it.

• Clusters cannot contain LOBs, either as key or nonkey columns.
• Even though compressed VARRAY data types are supported, they are less performant.
• The following data structures are supported only as temporary instances. You cannot store these instances in database tables:
  • VARRAY of any LOB type
  • VARRAY of any type containing a LOB type, such as an object type with a LOB attribute
  • ANYDATA of any LOB type
  • ANYDATA of any type containing a LOB
• The first (INITIAL) extent of a LOB segment must contain at least three database blocks.
• The minimum extent size is 14 blocks. For an 8K block size (the default), this is equivalent to 112K.
• When creating an AFTER UPDATE DML trigger, you cannot specify a LOB column in the UPDATE OF clause. For a table on which you have defined an AFTER UPDATE DML trigger, if you use OCI functions or the DBMS_LOB package to change the value of a LOB column or the LOB attribute of an object type column, the database does not fire the DML trigger.
• You cannot specify a LOB column as part of an index key. However, you can specify a LOB column in the indextype specification of a functional or domain index. In addition, Oracle Text lets you define an index on a CLOB column.
• In SQL Loader, a field read from a LOB cannot be used as an argument to a clause.
• Case-insensitive searches on CLOB columns often do not succeed. If you perform the following case-insensitive search on a CLOB column:

  ALTER SESSION SET NLS_COMP=LINGUISTIC;
  ALTER SESSION SET NLS_SORT=BINARY_CI;
  SELECT * FROM ci_test WHERE LOWER(clob_col) LIKE 'aa%';
  

  The select fails without the LOWER function. You can perform case-insensitive searches with Oracle Text or the DBMS_LOB.INSTR() function.