This class has a very important role in SELECT
performance with select_result class and
classes inherited from it (usually called with a
select_ prefix). This class provides the
interface for transmitting results.
The key methods in this class are the following:
send_fields sends given item list headers
(type, name, etc.).
send_data sends given item list values as
row of table of result.
send_error is used mainly for error
interception, making some operation and then
::send_error will be called.
For example, there are the following
select_result classes:
select_send used for sending results though
network layer.
select_export used for exporting data to
file.
multi_delete used for multi-delete.
select_insert used for
INSERT ... SELECT ...
multi_update used for multi-update.
select_singlerow_subselect used for row and
scalar subqueries..
select_exists_subselect used for
EXISTS/IN/ALL/ANY/SOME
subqueries.
select_max_min_finder_subselect used for
min/max subqueries
(ALL/ANY subquery
optimization).
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