If you export the table after designating a column as unused,
the column will not be exported.
You can drop multiple columns in a single command, as shown in the following example:
alter table TABLE1 drop (Col4, Col5);
NOTE
When dropping multiple columns, you should not use the column
keyword of the alter table command. The multiple column names
must be enclosed in parentheses, as shown in the preceding example.
If the dropped columns are part of primary keys or unique constraints, you will also need to
use the cascade constraints clause as part of your alter table command. If you drop a column that
belongs to a primary key, Oracle will drop both the column and the primary key index.
If you cannot immediately arrange for a maintenance period during which you can drop
the columns, mark them as unused. During a later maintenance period, you can complete the
maintenance from the SYS or SYSTEM account.
Forcing Cursor Sharing
Ideally, application developers should use bind variables in their programs to maximize the reuse
of their previously parsed commands in the shared SQL area. If bind variables are not in use, you
may see many very similar statements in the library cache??”queries that differ only in the literal
value in the where clause.
160 Oracle Database 11g DBA Handbook
Statements that are identical except for their literal value components are called similar
statements. Similar statements can reuse previously parsed commands in the shared SQL area
if the CURSOR_SHARING initialization parameter is set to SIMILAR or FORCE.
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