Oracle 中使用子查询和文字表达式的 SQL 性能

Question

I am encounter some performance issue for following scenario and want to ask what could be the problem from performace perspective because I think those to queries should be run cca. same time.

imagine following 2 queries, the queries are much more longer in reality but for simplicity I reduced them

select * from table where x = 2010;

select * from table where x = (select year from yearTable where ID=1234);

there is an index on column X and the subquery gives exact one year 2010 but this query runs forever compared to the first one with literal (few seconds )

from technical perspective it should not be much difference between them. Of course the subquery has to be evaluated but this is in milliseconds cause there is an index on ID as well.

I found out that with this issue , literal vs subquery , fighting severeal SQL statments in the database.

what could be the problem?

thanks

Answer 1

For the second query:

SELECT *
FROM table
WHERE x = (SELECT year FROM yearTable WHERE ID = 1234);

you actually want to index the table involved in the subquery for fast lookup:

CREATE INDEX idx1 ON yearTable (ID, year);

Adding an index on x to the table in the outer query likely won't do much, assuming that Oracle would choose to scan this table.