View Performance - oracle

Question

I have requirement of performing some calculation on a column of a table with large date set ( 300 GB). and return that value.
Basically I need to create a View on that table. Table has data of 21 years and It is partitioned on date column (Daily). We can not put date condition on View's query and User will put filter on runtime while execution of the view.
For example:
Create view v_view as
select * from table;
Noe I want to query View like
Select * v_view where ts_date between '1-Jan-19' and '1-Jan-20'
How Internally Oracle execute above statement? Will it execute view query first and then put date filter on that?
If so will there not be performance issue ? and how to resolve this?

Answer 1

oracle first generates the view and then applies the filter. you can create a function that input may inserted by user. the function results a create query and if yo run the query then the view will be created. just run:
create or replace function fnc_x(where_condition in varchar2)
return varchar2
as
begin
return ' CREATE OR REPLACE VIEW sup_orders AS
SELECT suppliers.supplier_id, orders.quantity, orders.price
FROM suppliers
INNER JOIN orders
ON suppliers.supplier_id = orders.supplier_id
'||where_condition||' ';
end fnc_x;
this function should be run. input the function is a string like this:
''WHERE suppliers.supplier_name = Microsoft''
then you should run a block like this to run the function's result:
cl scr
set SERVEROUTPUT ON
declare
szSql varchar2(3000);
crte_vw varchar2(3000);
begin
szSql := 'select fnc_x(''WHERE suppliers.supplier_name = Microsoft'') from dual';
dbms_output.put_line(szSql);
execute immediate szSql into crte_vw; -- generate 'create view' command that is depended on user's where_condition
dbms_output.put_line(crte_vw);
execute immediate crte_vw ; -- create the view
end;
In this manner, you just need received where_condition from user.

Answer 2

Oracle can "push" the predicates inside simple views and can then use those predicates to enable partition pruning for optimal performance. You almost never need to worry about what Oracle will run first - it will figure out the optimal order for you. Oracle does not need to mindlessly build the first step of a query, and then send all of the results to the second step. The below sample schema and queries demonstrate how only the minimal amount of partitions are used when a view on a partitioned table is queried.
--drop table table1;
--Create a daily-partitioned table.
create table table1(id number, ts_date date)
partition by range(ts_date)
interval (numtodsinterval(1, 'day'))
(
partition p1 values less than (date '2000-01-01')
);
--Insert 1000 values, each in a separate day and partition.
insert into table1
select level, date '2000-01-01' + level
from dual
connect by level <= 1000;
--Create a simple view on the partitioned table.
create or replace view v_view as select * from table1;
The following explain plan shows "Pstart" and "Pstop" set to 3 and 4, which means that only 2 of the many partitions are used for this query.
--Generate an explain plan for a simple query on the view.
explain plan for
select * from v_view where ts_date between date '2000-01-02' and date '2000-01-03';
--Show the explain plan.
select * from table(dbms_xplan.display(format => 'basic +partition'));
Plan hash value: 434062308
-----------------------------------------------------------
| Id | Operation | Name | Pstart| Pstop |
-----------------------------------------------------------
| 0 | SELECT STATEMENT | | | |
| 1 | PARTITION RANGE ITERATOR| | 3 | 4 |
| 2 | TABLE ACCESS FULL | TABLE1 | 3 | 4 |
-----------------------------------------------------------
However, partition pruning and predicate pushing do not always work when we may think they should. One thing we can do to help the optimizer is to use date literals instead of strings that look like dates. For example, replace
'1-Jan-19' with date '2019-01-01'. When we use ANSI date literals, there is no ambiguity and Oracle is more likely to use partition pruning.