Acceleration of NCLOB compression

  Hello!

in my Lob-table I see this datae allocation (Oracle 12.2)

select

  dbms_lob.getlength(source_data) bytes, count(1) distinct 

  from user1.Lob_table

  group by dbms_lob.getlength(source_data)

order by 2 desc

BYTES      DISTINCT

---------      -------------

....

385               671767

295	965257
	9051575

If I correctly understand,

- most part of column contains only locators without secure files ?

- in my table 9051575 filds become empty after DML-operations ?

- what can I do to accelerate very long  "alter table" operation

  "alter table user1.Lob_table modify lob(source_data) (compress high);" ?

regards,

Paul