Note: Over-allocated space is a very common problem in many applications where explosive data growth occurs and is not just an Ignite issue.

Ignite detail tables can grow very quickly when a large performance anomaly occurs.  When an anomaly occurs, more data is typically collected by Ignite than normal causing the data bloat.  By default after 30 days, Ignite will summarize the detailed data into summary tables for long term storage and purge the bloated detailed data.  However, purging the data originating from the performance anomoly does not shrink the underlying tables and you can end up with wasted space.

At the bottom of this article is a script that can be used to detect this problem and provides a ranking of the tables with the most data bloat, i.e. the tables that have the most unused space.  To shrink a table and reclaim this wasted space, you can use the following commands:

alter table <table_name> enable row movement;
alter table <table_name> shrink space;
alter table <table_name> disable row movement;

This is just one way to shrink the table, but is probably the easiest.  Other methods include:
- create table as select (CTAS). However, you woulud also have to manually rebuild the indexes and then rename objects at the end
- Online regorganization with DBMS_REDEFINITION package.
- alter table move
- export/import and data pump (expdp/impdp)

Script to find over-allocated tables

drop table ignite_space
/
create table ignite_space as
select owner, table_name, 0 space_used, 0 space_alloc, 0 chain_pct
from dba_tables where 1=2
/
declare
   nSpace_Used    NUMBER;
   nSpace_Alloc    NUMBER;
   nChain_Pct    NUMBER;
begin
   for t in (select user owner, segment_name table_name from user_segments where segment_type='TABLE') loop
      begin
         dbms_space.object_space_usage (t.owner, t.table_name, 'TABLE', NULL, nSpace_Used, nSpace_Alloc, nChain_Pct);
         insert into ignite_space (owner, table_name, space_used, space_alloc, chain_pct)
         values (t.owner, t.table_name, nSpace_Used, nSpace_Alloc, nChain_Pct);
         commit;
      exception
         when others then
         insert into ignite_space (owner, table_name, space_used, space_alloc, chain_pct)
         values (t.owner, t.table_name, -1, -1, -1);
         commit;
      end;
   end loop;
end;
/
-- show top 20 space savers if shrunk
select * from (
   select table_name,
          round((space_alloc-space_used)/1024/1024,2) mb_save,
          round(space_alloc/1024/1024,2) mb_alloc,
          round(space_used/1024/1024,2) mb_used,
          round(chain_pct,2) chain_pct
   from ignite_space
   order by 2 desc)
where rownum<=20
/