How to drop columns in a compressed table?

Question

After compressing a table, it's not possible to drop a column any more:

CREATE TABLE foo (p NUMBER, i NUMBER, j NUMBER) COMPRESS BASIC;

ALTER TABLE foo DROP COLUMN j;
ORA-39726: unsupported add/drop column operation on compressed tables

It is possible if advance compression is used:

CREATE TABLE foo (p NUMBER, i NUMBER, j NUMBER) COMPRESS FOR OLTP;

ALTER TABLE foo DROP COLUMN j;
Table FOO altered.

However, the column is not really dropped, just hidden:

SELECT column_name, data_type, hidden_column 
  FROM user_tab_cols WHERE table_name = 'FOO';

COLUMN_NAME                 DATA_TYPE  HIDDEN_COLUMN
P                           NUMBER     NO
I                           NUMBER     NO
SYS_C00002_18030204:09:26$  NUMBER     YES

This causes an error when partitions are exchanged:

CREATE TABLE par (p NUMBER, i NUMBER) 
 PARTITION BY LIST(p)(
 PARTITION p1 VALUES(1), 
 PARTITION p2 VALUES(2)
);

ALTER TABLE par EXCHANGE PARTITION p1 WITH TABLE foo;
ORA-14097: column type or size mismatch in ALTER TABLE EXCHANGE PARTITION

How can the hidden column be removed? I tried

ALTER TABLE foo DROP UNUSED COLUMNS;

but it doesn't help:

SELECT column_name, data_type, hidden_column 
  FROM user_tab_cols WHERE table_name = 'BAR';

COLUMN_NAME                 DATA_TYPE  HIDDEN_COLUMN
P                           NUMBER     NO
I                           NUMBER     NO
SYS_C00002_18030204:09:26$  NUMBER     YES

Answer 1

Oracle support document 1987500.1 "How to Drop Columns in Compressed Tables" had the solution: the table needs to be uncompressed first, the the columns removed, then it can be compressed again:

ALTER TABLE foo MOVE NOCOMPRESS;
ALTER TABLE foo DROP UNUSED COLUMNS;
ALTER TABLE foo MOVE COMPRESS FOR OLTP;

SELECT column_name, data_type, hidden_column 
  FROM user_tab_cols WHERE table_name = 'FOO';

COLUMN_NAME                 DATA_TYPE  HIDDEN_COLUMN
P                           NUMBER     NO
I                           NUMBER     NO

ALTER TABLE par EXCHANGE PARTITION p1 WITH TABLE foo;
Table PAR altered.

N.B. The compression/recompression is fast if the table is empty.

Answer 2

update foo set j = null;
commit;
alter table foo set unused column j;

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