oracle select date from partition table - oracle

Question

Following statement allows me to retrieve information about a table's partitions:
select table_name, partition_name, high_value from user_tab_partitions where table_name = 'T1';
problem is that, for some unknown reason, high_value column's values are expressed as:
TO_DATE(' 2015-01-01 00:00:00', 'SYYYY-MM-DD HH24:MI:SS', 'NLS_CALENDAR=GREGORIAN')
is it possible to retrieve them as date? Or at least do something like "eval" on that expression?

Answer 1

That field is defined as a long type, so depending on the partitioning scheme, it can hold various values (a string like 'ABC', a number 123, a string representation of a date, whatever).
Anyway, sounds like you want to get data from a particular partition, and you're trying to figure out the partition name from the metadata. If thats the case, you can simply use the partition for clause:
select * from my_partitioned_table partition for (to_date('20150801', 'YYYYMMDD'));
Which would select data from the partition housing that particular date (assuming your partitioning by date). This is particularly useful in interval partitioning, where Oracle assigns partition names like SYS_xxxx which seem arbitrary at best.
If you'd like to drop partitions older than a given date, its a bit more tricky. The above syntax is for selecting data, not DDL (alter table). To do that, you could do something like this (loosely tested):
Create a function to identify which partitions hold data with dates less than a given reference date:
create or replace function fn_partition_is_earlier(i_part_tab_name varchar2, i_partition_position number, i_ref_date in date)
return number
is
l_date_str varchar2(2000);
l_date date;
begin
execute immediate 'select high_value from all_tab_partitions where table_name = :tab and partition_position = :pos'
into l_date_str
using i_part_tab_name, i_partition_position;
execute immediate 'select ' || l_date_str || ' from dual' into l_date;
if (l_date < i_ref_date) then
return 1;
end if;
return 0;
end;
Use the function as follows:
with part_name as (
select partition_name
from (
select fn_partition_is_earlier(p.table_name, p.partition_position, to_date('20130501', 'YYYYMMDD')) should_drop_flag, p.*
from all_tab_partitions p
where table_name = 'MY_TAB'
)
where should_drop_flag = 1
)
select 'alter table MY_TAB drop partition ' || part_name.partition_name || ' update global indexes;'
from part_name;
The output would give you the script you'd have the DBAs run off hours.
Hope that helps.