Select from several partitions at once

Question

Excuse me for my english. I have 2 tables, both partitioned by date interval, but on different fields. There is a big amount of records in both tables(~100kk in each partition). First table keep it's 3 last(by date) partitions in fast discks tablespace, others partitions is in slow discks tablespace. Also I have some system, which processing data. It execute processes in parallel, each one get data from first table by select statement and put processed data into second table. So I need select data from first table only from "fast"(!) partitions to put it in second table. But second table partitioned on other(date too) field. And when processes executing in parallel I get deadlocks when different processes trying to put data into the same partition in 2nd table.

Good solution is for each process take data from only "fast" partitions(but all of them in one time) only data for one partition in 2nd table. In this case each process will push data in one partition. But I don't have any idea how to do it.

If I make

select t.field1, t.field2 from (select * from FIRST_TABLE partition("P1") union all
select * from FIRST_TABLE partition("P2") union all
select * from FIRST_TABLE partition("P3")) t
where t.field3='someVal' --Indexed field in FIRST_TABLE 

will OracleDB use local indexes on partitions in FIRST_TABLE to resolve where-clause? How will this way affect the performance?

Any ideas to solve my problem?

PS It's a lot of questions about how to select data from several partitions in one select-statement, but I didn't found answer usefull for my situation.

Answer 1

You almost never want to use the PARTITION clause when querying a partitioned table. You almost always want to specify a predicate that allows Oracle to do partition pruning on its own.

SELECT t.column1, t.column2
  FROM first_table t
 WHERE t.partitioned_date_column >= <<date that delimits fast partitions>>
   AND t.column3 = 'someVal'

When you specify a predicate on the date column that the table is partitioned on, Oracle can automatically determine which partition(s) need to be accessed.

Answer 2

you can get deadlock if you are trying to execute three queries in parallel on your own, for example running at the same time:

insert into t2 select from t1 partition ("P1");

and then in another shell/window/job:

insert into t2 select from t1 partition ("P2")

If you query

select *
from t1
where date_column_used_for_partition >= 3_dates_ago

Oracle will select only the three partitions that you need and you won't need to use UNION.

In this way you can put your INSERT INTO... SELECT statement in a single query and you don't need to worry about deadlock, the Oracle engine will know in which partition of the second table it has to insert the data and he will manage the inserts for you.