pl sql declare variable with query too - oracle

Question

I want to make a simple query in pl sql
Please suggest and how to make it MORE FAST EXECUTE (maybe only 0.01 second in 1000000 data)
first query:
select datetime
from product
order by datetime desc
FETCH NEXT 1 ROWS ONLY
Result of first query will be used in second query.
select *
from traceability
where endtime = [first query]
Please help me to implement that logic to pl sql
Thank you.

Answer 1

Please find bellow an example with sample data.
create table product as
select rownum product_id, DATE'2020-01-01' + NUMTODSINTERVAL(rownum-1, 'second') datetime
from dual connect by level <= 10;
create index product_idx on product(datetime);
create table traceability as
select
rownum id, DATE'2020-01-01' + NUMTODSINTERVAL(rownum-1, 'second') endtime
from dual connect by level <= 10;
create index traceability_idx on traceability(endtime);
Your query shou be as follows
select *
from traceability
where endtime =
(select max(datetime)
from product );
The query will lead to this execution plan. See here how to get the execution plan.
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 22 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID | TRACEABILITY | 1 | 22 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | TRACEABILITY_IDX | 1 | | 1 (0)| 00:00:01 |
| 3 | SORT AGGREGATE | | 1 | 9 | | |
| 4 | INDEX FULL SCAN (MIN/MAX)| PRODUCT_IDX | 1 | 9 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ENDTIME"= (SELECT MAX("DATETIME") FROM "PRODUCT" "PRODUCT"))
Note that in case that in the table TRACEABILITY will be a large number of rows with the max timestamp, you can also see a FULL TABLE SCAN in the line 1.
Similar is valid for the PRODUCT table and the line 4