Select statement modifies blocks in two situations: first when the option for update is specified,and second when deferred block cleanout occurs

to store fewer rows per block ,either a higher pctfree or a smaller block size can be used

the goal of freelist is to spread concurrent insert statements over several blocks

To have data compression at the table level . Following condition must be full fulled

1) The table cannot have more than 255 columns
2) The table cannot be created with parameter rowdependencies

Free space in the compressed block caused by delete statements is never used

Data compression should be used for read-only table

Data Marts and Completely refreshed materialzed  views are good candidate for data compression