How inefficient are virtual table JOINs?

Say I have a query like this, where I join a number of virtual tables:

SELECT table1.a, tbl2.a, tbl3.b, tbl4.c, tbl5.a, tbl6.a
FROM table1
JOIN (SELECT x, a, b, c FROM table2 WHERE foo='bar') tbl2 ON table1.x = tbl2.x
JOIN (SELECT x, a, b, c FROM table3 WHERE foo='bar') tbl3 ON table1.x = tbl3.x
JOIN (SELECT x, a, b, c FROM table4 WHERE foo='bar') tbl4 ON table1.x = tbl2.x
JOIN (SELECT x, a, b, c FROM table5 WHERE foo='bar') tbl5 ON table1.x = tbl5.x
JOIN (SELECT x, a, b, c FROM table6 WHERE foo='bar') tbl6 ON table1.x = tbl6.x
WHERE anotherconstraint='value'

In my real query, each JOIN has its own JOINs, aggregate functions, and WHERE constraints.

How well/poorly would a query like this run? Also, what is the impact difference between this and running all of the individual virtual tables as their 开发者_运维百科own query and linking the results together outside of SQL?

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There's nothing inherently bad about using inline views (which is AFAIK the correct term for what you call "virtual tables"). I do recommend learning to view and understand execution plans so you can investigate specific performance issues.

In general, I think it's a very bad idea to execute multiple single-table queries and then essentially join the results together in your front-end code. Doing joins is what an RDBMS is designed for, why re-write it?

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Why not just:

SELECT table1.a, tbl2.a, tbl3.b, tbl4.c, tbl5.a, tbl6.a
FROM table1 JOIN table2 on table1.x = table2.x AND table2.foo = 'bar'
            JOIN table3 on table1.x = table3.x AND table3.foo = 'bar'
            JOIN table4 on table1.x = table4.x AND table4.foo = 'bar'
            JOIN table5 on table1.x = table5.x AND table5.foo = 'bar'
            JOIN table6 on table1.x = table6.x AND table6.foo = 'bar'
WHERE anotherconstraint='value';

EDIT:

How well would it run? Who knows? As @Vinko states, the answer lies in looking at the execution plan, perhaps supplying hints where appropriate. Something this complex cannot be answered by looking at a contrived example.