Question

We have tables with more then 3m records. When using innerjoin it is much slower then select * from db1,db2 where db1.field=db2.field

Any thoughts?

Answer 1

SELECT * from t1, t2 where t1.id = t2.id

is equivalent to

SELECT * from t1 INNER JOIN t2 on t1.id = t2.id.

However, if there are other criteria for the SQL query, then the behaviour may differ. For instance.

SELECT * from t1, t2 where t1.id = t2.id and t1.col1 is not null;

can be written in two different ways with the INNER JOIN:

SELECT * from t1 INNER JOIN t2 on t1.id = t2.id and t1.col1 is not null

or

SELECT * from t1 INNER JOIN t2 on t1.id = t2.id
WHERE t1.col1 is not null

This may or may not end up being the same query (according to the optimiser), and the complexity of the other parts of the query. The EXPLAIN PLAN will tell you if you are executing the same query.

Why are the above queries different? Because the restriction on not null is done at different stages of the query, which may have an impact on the performance, or even on the number of rows returned.