Possible perf issue: handler objects are created for merged derived tables

This is a continuation of MDEV-7586.

If one takes a basic testcase with a merged derived table (or a view):

create table t2 (a int);

insert into t2 values (1),(2),(3);

create view v2 as select a from t2;

select * from v2;

they can see that the code calls create_tmp_table(... do_not_open=true. ...) for the derived table (or the merged view). This may add extra overhead (although we are not sure how much ATM).

This effect was introduced in MariaDB 5.3, in this commit:

igor@askmonty.org-20100526201818-en2jqo1p1t3hi3ra

Igor Babaev <igor@askmonty.org>

maria-5.3-mwl106

Wed 2010-05-26 13:18:18 -0700

MWL#106: Backport optimizations for derived tables and views.

The main consolidated patch.

MySQL 5.6 doesn't have this optimization, so they dont have this problem.