The optimizer removes redundant GROUP BY operations. If GROUP BY element
is a subselect, it is "eliminated".
However one must not eliminate the item if it is used both in the select
list and in the GROUP BY, like so:
select (select ... ) as SUBQ from ... group by SUBQ
Do not eliminate such items.
Allow materialization strategy when collations on the
inner and outer sides of an IN subquery are the same and the
character set of the inner side is a proper subset of the character
set on the outer side.
This allows conversion from utf8mb3 to utf8mb4
as the former is a subset of the later.
This is only allowed when IN predicate is converted to an IN subquery
Backported part of the patch (d6a00d9b18) of MDEV-17905.
When duplicates are removed from a table using a hash, if the record is a duplicate it is marked
as deleted. The handler API check if the record is deleted and send an error flag HA_ERR_RECORD_DELETED.
When we scan over the table if the thread is not killed then we skip the
records marked as HA_ERR_RECORD_DELETED.
The issue here is when a query is aborted by a user (this is happening when the LIMIT for ROWS EXAMINED
is exceeded), the scan over the table does not skip the records for which HA_ERR_RECORD_DELETED is sent.
It just returns an error flag HA_ERR_ABORTED_BY_USER.
This error flag is not checked at the upper level and hence we hit the assert.
If the query is aborted by the user we should just skip reading rows and return
control to the upper levels of execution.
Step #2: "[ORDER BY ...] LIMIT n" should not prevent EXISTS-to-IN
conversion, as long as
- the LIMIT clause doesn't have OFFSET
- the LIMIT is not "LIMIT 0".
Step 1: Removal of ORDER BY [LIMIT] from the subquery should be done
earlier and for broader class of subqueries.
The rewrite was done in Item_in_subselect::select_in_like_transformer(),
but this had problems:
- It didn't cover EXISTS subqueries
- It covered IN-subqueries, but was done after the semi-join transformation
was considered inapplicable, because ORDER BY was present.
Remaining issue:
- EXISTS->IN transformation happens before
check_and_do_in_subquery_rewrites() is called, so it is still prevented
by the present ORDER BY.
