My question is similar to this SQL order of operations but with a little twist, so I think it's fair to ask.
I'm using Teradata. And I have 2 tables: table1, table2.
table1 has only an id column.
table2 has the following columns: id, val
I might be wrong but I think these two statements give the same results.
Statement 1.
SELECT table1.id, table2.val
FROM table1
INNER JOIN table2
ON table1.id = table2.id
WHERE table2.val<100
Statement 2.
SELECT table1.id, table3.val
FROM table1
INNER JOIN (
SELECT *
FROM table2
WHERE val<100
) table3
ON table1.id=table3.id
My questions is, will the query optimizer be smart enough to
- execute the WHERE clause first then JOIN later in Statement 1
- know that table 3 isn't actually needed in Statement 2
I'm pretty new to SQL, so please educate me if I'm misunderstanding anything.
Answer
this would depend on many many things (table size, index, key distribution, etc), you should just check the execution plan:
you don't say which database, but here are some ways:
MySql EXPLAIN
SQL Server SET SHOWPLAN_ALL (Transact-SQL)
Oracle EXPLAIN PLAN
what is explain in teradata?
Teradata Capture and compare plans faster with Visual Explain and XML plan logging