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Fixed MDEV-13994 Bad join results with orderby_uses_equalities=on.

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This patch effectively blocks the optimization that uses multiple
equalities for ORDER BY to remove tmp table in the case when
the first table happens to be the result of materialization of
a semi-join nest. Currently there is no code at the execution level
that would support the optimization in this case.
This commit is contained in:
Igor Babaev
2017-11-10 13:58:00 -08:00
parent 589b0b3655
commit dcbf2823c7
5 changed files with 241 additions and 1 deletions
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31
sql/sql_select.cc
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@ -12519,8 +12519,37 @@ remove_const(JOIN *join,ORDER *first_order, COND *cond,
can be used without tmp. table.
*/
bool can_subst_to_first_table= false;
bool first_is_in_sjm_nest= false;
if (first_is_base_table)
{
TABLE_LIST *tbl_for_first=
join->join_tab[join->const_tables].table->pos_in_table_list;
first_is_in_sjm_nest= tbl_for_first->sj_mat_info &&
tbl_for_first->sj_mat_info->is_used;
}
/*
Currently we do not employ the optimization that uses multiple
equalities for ORDER BY to remove tmp table in the case when
the first table happens to be the result of materialization of
a semi-join nest ( <=> first_is_in_sjm_nest == true).
When a semi-join nest is materialized and scanned to look for
possible matches in the remaining tables for every its row
the fields from the result of materialization are copied
into the record buffers of tables from the semi-join nest.
So these copies are used to access the remaining tables rather
than the fields from the result of materialization.
Unfortunately now this so-called 'copy back' technique is
supported only if the rows are scanned with the rr_sequential
function, but not with other rr_* functions that are employed
when the result of materialization is required to be sorted.
TODO: either to support 'copy back' technique for the above case,
or to get rid of this technique altogether.
*/
if (optimizer_flag(join->thd, OPTIMIZER_SWITCH_ORDERBY_EQ_PROP) &&
first_is_base_table &&
first_is_base_table && !first_is_in_sjm_nest &&
order->item[0]->real_item()->type() == Item::FIELD_ITEM &&
join->cond_equal)
{