It's a good solution - question about UNION sulution

Question

As I write in subject:
SELECT serial_no, model_no, timestamp2
FROM TABLE_XXX WHERE serial_no = 7777 AND
model_no IS NOT NULL
UNION
SELECT serial_no, part_no model_no, timestamp2
FROM TABLE_YYY serial_no = 7777 AND
part_no LIKE 'WD%'
UNION
SELECT serial_no, model_no, timestamp2
FROM TABLE_ZZZ WHERE serial_no = 7777 AND
model_no IS NOT NULL
UNION
SELECT serial_no, model_no, timestamp2
FROM TEST_WWW WHERE serial_no = 7777 AND
model_no IS NOT NULL;
Please expalin why yes/no ??!!

Answer 1

And I reflect that if I try to wreck this query and divide to smaller part then this select will be more speeder??!! What do you think??
Why? If the database has to read X Blocks to get all the data you need the number of blocks does not change only because you split the SQL in smaller fragments.
Which plan does oracle use? How much rows are in the tables and hoe are the cardinalities of the rows in the WHERE clause?
Are the tables and indexes analyzed using the dbms_stats?
explain plan for SELECT serial_no, model_no, timestamp2
FROM TABLE_XXX WHERE serial_no = 7777 AND
model_no IS NOT NULL
UNION
SELECT serial_no, part_no model_no, timestamp2
FROM TABLE_YYY serial_no = 7777 AND
part_no LIKE 'WD%'
UNION
SELECT serial_no, model_no, timestamp2
FROM TABLE_ZZZ WHERE serial_no = 7777 AND
model_no IS NOT NULL
UNION
SELECT serial_no, model_no, timestamp2
FROM TEST_WWW WHERE serial_no = 7777 AND
model_no IS NOT NULL;
select * from table(dbms_xplan.display);
select table_name,num_rows from user_tables where table_name in('TABLE_ZZZ','TEST_WWW',...);Dim