Question

I have a query which includes LIMIT to the main table and JOIN.

My questions is that: what is coming before? the query finds the x rows of the LIMIT and then doing JOIN to these rows or doing first the JOIN on the all rows and just after that LIMIT?

Answer 1

LIMIT Applies to the query to which it is applied. It will be applied to the query AFTER the JOINs in that query, but if the derived table is JOINed to other tables, that/those JOIN(s) comes after.

e.g.

SELECT ..
  FROM (SELECT ..
          FROM TABLE1 T1
          JOIN TABLE2 T2 ON ..
         LIMIT 10) X
  JOIN OTHERTABLE Y
 LIMIT 20;

• The JOIN between T1 and T2 occurs first
• LIMIT 10 is applied to result from the previous step, so only 10 records from this derived table will be used in the outer query
• LIMIT 20 is applied to the result of the JOIN between X and Y

Although LIMIT is a specific keyword for PostgreSQL, MySQL and SQLite, the TOP keyword and processing in SQL Server works the same way.