does a subquery pull the entire table?

i am attempting to run a subquery in a table:

SELECT t1.*, t2.* FROM t1 
LEFT JOIN (SELECT * FROM table2 GROUP BY col1) AS t2 on t1.col1 = t2.col1
WHERE ...

My main question is, since that subquery called on it's own would pull the entire time, does it also pull the entire time while it's in a subquery, even though the WHERE condition on the parent query is only pulling just a few rows of the actual database?

I suppose I could put the where query inside the subquery, but its a pretty complex where query that involves other tables.

It shouldn't. It depends on what the system can simplify the query to.

You can demonstrate this very easily by considering the plan for queries such as:

SELECT *
FROM YourTable
WHERE ID < 5;

SELECT *
FROM (SELECT * FROM YourTable) as t
WHERE ID < 5;

The two should be identical.

StackExchange Database Administrators Q#33426, answer score: 3