Oracle在SQL查询中使用变量

Question

I have something like this:

With a as (select id from table_a)
select * from table_b where table_b.id > (select min(id) from a)

table_a is a huge table with millions of records and I don't want to go through all the records to find the min(i) every time I want to use min(i). Is there any way for me to store the min(id) in a variable and use it in the query? Something like this :

With a as (select id from table_a),
b as ((select min(id) into min_id from a))
select * from table_b where table_b.id > min_id

Answer 1

Your query should be doing what you want. But, if you really want to be sure, move the subquery to the from clause:

select b.*
from table_b b join
     (select min(id) as min_id from a) a
     on b.id > a.min_id;

Answer 2

You may help Oracle by defining an index on the table_a on column id to not go through all the records to find the min .

If the index is defined, Oracle performs a single index access to get the minimal row.

Without the index, you'll hav eto perform one FULL TABLE SCAN of the large table.

The best way to verify is to observe the execution plan:

 create index idx_a on table_a(id);  

EXPLAIN PLAN  SET STATEMENT_ID = 'q1' into   plan_table  FOR
With a as (select id from table_a)
select * from table_b 
where table_b.id > (select min(id) from a);
SELECT * FROM table(DBMS_XPLAN.DISPLAY('plan_table', 'q1','ALL')); 

---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |  5524 | 71812 |    49   (3)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL          | TABLE_B |  5524 | 71812 |    47   (3)| 00:00:01 |
|   2 |   SORT AGGREGATE            |         |     1 |    13 |            |          |
|   3 |    INDEX FULL SCAN (MIN/MAX)| IDX_A   |     1 |    13 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("TABLE_B"."ID"> (SELECT MIN("ID") FROM "TABLE_A" "TABLE_A"))