Question

I am not sure if this question is an obvious one. I need to delete a load of data. Delete is expensive. I need to truncate the table but not fully so that the memory is released and watermark is changed.

Is there any feature which would allow me to truncate a table based on a condition for select rows?

Answer 1

Depends on how your table is organised.

1) if your (large) table is partitioned based on similar condition ( eg. you want to delete previous month’s data and your table is partitioned by month), you could truncate only that partition, instead of the entire table.

2) The other option, provided you have some downtime, would be to insert the data that you want to keep into a temporary table, truncate the original table and then load the data back.

insert into <table1>
select * from <my_table>
where <condition>;

commit;

truncate table my_table;

insert into my_table
select * from <table1>;

commit;

--since the amount of data might change considerably, 
--you might want to collect statistics again

exec dbms_stats.gather_table_stats
     (ownname=>'SCHEMA_NAME', 
      tabname => 'MY_TABLE');