MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
The optimizer handles derived tables as follows:
The optimizer postpones materialization of derived tables until their contents are needed during query execution, which improves performance.
For non-EXPLAIN
queries,
delay of materialization may result in not having to do it
at all. Consider a query that joins the result of a
derived table to another table: If the optimizer processes
that other table first and finds that it returns no rows,
the join need not be carried out further and the optimizer
can completely skip materializing the derived table.
During query execution, the optimizer may add an index to a derived table to speed up row retrieval from it.
Consider the following EXPLAIN
statement, for a SELECT
query
that contains a derived table:
EXPLAIN SELECT * FROM (SELECT * FROM t1) AS derived_t1;
The optimizer avoids materializing the derived table by
delaying it until the result is needed during
SELECT
execution. In this case,
the query is not executed (because it occurs in an
EXPLAIN
statement), so the
result is never needed.
Even for queries that are executed, delay of derived table materialization may enable the optimizer to avoid materialization entirely. Consider the following query, which joins the result of a derived table to another table:
SELECT * FROM t1 JOIN (SELECT t2.f1 FROM t2) AS derived_t2 ON t1.f2=derived_t2.f1 WHERE t1.f1 > 0;
If the optimization processes t1
first and
the WHERE
clause produces an empty result,
the join must necessarily be empty and the derived table need
not be materialized.
For cases when a derived table requires materialization, the
optimizer may add an index to the materialized table to speed
up access to it. If such an index enables
ref
access to the table, it
can greatly reduce amount of data read during query execution.
Consider the following query:
SELECT * FROM t1 JOIN (SELECT DISTINCT f1 FROM t2) AS derived_t2 ON t1.f1=derived_t2.f1;
The optimizer constructs an index over column
f1
from derived_t2
if
doing so would enable use of
ref
access for the lowest
cost execution plan. After adding the index, the optimizer can
treat the materialized derived table the same as a regular
table with an index, and it benefits similarly from the
generated index. The overhead of index creation is negligible
compared to the cost of query execution without the index. If
ref
access would result in
higher cost than some other access method, the optimizer
creates no index and loses nothing.
For optimizer trace output, a merged derived table or view reference is not shown as a node. Only its underlying tables appear in the top query's plan.