The basic problem here is that we sometimes attempt to set the keyread
flag on a temporary table that has not been instantiated yet.
This is acceptable for a Memory temporary table, but not for a MyISAM
temporary table. The solution for this problem is to save the keyread
flag in the TABLE object and forward it to the storage engine when
the table is instantiated. To prevent misuse, the "created" flag is made
private, and the set_created() function is introduced.

During debugging, it was also found that tab->keys was not always set
for a derived table, meaning that we sometimes missed that we had decided
to use a generated key. The problematic code was the test for
tab->keys.is_clear_all() in JOIN::set_access_methods(). This test has
been replaced with debug asserts (a selected key must be a candidate
key). Opportunity was also taken to approve readability of code in
JOIN::set_access_methods().

Fixing this problem meant some test case changes - earlier table scans
were converted to key lookups.

Removal of unused keys for derived tables used to update tab->ref.key,
but failed to add the selected key to tab->keys. It seems more
consistent to update the keyuse objects and tab->keys, and then let
create_ref_for_key() make the "correct" key choice immediately.
 
mysql-test/r/derived.result
  Changed explain result due to use of generated key.

mysql-test/r/distinct.result
  Changed explain result due to use of generated key.

mysql-test/suite/opt_trace/r/filesort_pq.result
  Changed explain result due to use of generated key.

mysql-test/t/derived.test
  Added analyze table for test stability.

sql/records.cc
  Use of flag "created" replaced with function "is_created()".

sql/sql_base.cc
  Uses functions "is_created()" and "set_created()".

sql/sql_derived.cc
  Uses functions "is_created()" and "set_created()".

sql/sql_executor.cc
  Use of flag "created" replaced with function "is_created()".

sql/sql_optimizer.cc
  JOIN::drop_unused_derived_keys() has been modified so that when
  deleting unused derived keys, it adjusts the affected keyuse objects
  instead of tab->ref.key. It also sets correct keys into tab->keys.
  In JOIN::optimize(), JOIN::drop_unused_derived_keys() is now called
  before JOIN::set_access_methods(). This can be done because we now fix
  the keyuse objects that JOIN::set_access_methods() rely upon.
  Use of flag "created" replaced with function "is_created()".

sql/sql_select.cc
  Use of flag "created" replaced with function "is_created()".
  In JOIN::set_access_methods(), no longer tests the redundant function
  tab->keys.is_clear_all() (testing tab->position->key is sufficient).
  Also removed some redundant initializations (checked with asserts)
  and expanded the if test for table scan and semi-join loose scan into
  two separate code sections.
  Expanded some TAB characters in make_join_readinfo().

sql/sql_tmp_table.cc
  Setting of flag "created" replaced with functions "set_created()
  and set_deleted().

sql/table.h
  Made flag "created" private.
  Added functions is_created(), set_created() and set_deleted().
  In TABLE::set_keyread(), added check for is_created() before sending
  keyread flag to storage engine.