can be specified
Currently MySQL allows one to specify what indexes to ignore during
join optimization. The scope of the current USE/FORCE/IGNORE INDEX
statement is only the FROM clause, while all other clauses are not
affected.
However, in certain cases, the optimizer
may incorrectly choose an index for sorting and/or grouping, and
produce an inefficient query plan.
This task provides the means to specify what indexes are
ignored/used for what operation in a more fine-grained manner, thus
making it possible to manually force a better plan. We do this
by extending the current IGNORE/USE/FORCE INDEX syntax to:
IGNORE/USE/FORCE INDEX [FOR {JOIN | ORDER | GROUP BY}]
so that:
- if no FOR is specified, the index hint will apply everywhere.
- if MySQL is started with the compatibility option --old_mode then
an index hint without a FOR clause works as in 5.0 (i.e, the
index will only be ignored for JOINs, but can still be used to
compute ORDER BY).
See the WL#3527 for further details.
Currently in the ONLY_FULL_GROUP_BY mode no hidden fields are allowed in the
select list. To ensure this each expression in the select list is checked
to be a constant, an aggregate function or to occur in the GROUP BY list.
The last two requirements are wrong and doesn't allow valid expressions like
"MAX(b) - MIN(b)" or "a + 1" in a query with grouping by a.
The correct check implemented by the patch will ensure that:
any field reference in the [sub]expressions of the select list
is under an aggregate function or
is mentioned as member of the group list or
is an outer reference or
is part of the select list element that coincide with a grouping element.
The Item_field objects now can contain the position of the select list
expression which they belong to. The position is saved during the
field's Item_field::fix_fields() call.
The non_agg_fields list for non-aggregated fields is added to the SELECT_LEX
class. The SELECT_LEX::cur_pos_in_select_list now contains the position in the
select list of the expression being currently fixed.
When resolving unqualified name references MySQL was not
checking what is the item type for the reference. Thus
e.g a string literal item that has by convention a name
equal to its string value will also work as a reference to
a SELECT list item or a table field.
Fixed by allowing only Item_ref or Item_field to referenced by
(unqualified) name.
Currently SQL_BIG_RESULT is checked only at compile time.
However, additional optimizations may take place after
this check that change the sort method from 'filesort'
to sorting via index. As a result the actual plan
executed is not the one specified by the SQL_BIG_RESULT
hint. Similarly, there is no such test when executing
EXPLAIN, resulting in incorrect output.
The patch corrects the problem by testing for
SQL_BIG_RESULT both during the explain and execution
phases.
optimizer does not honor IGNORE INDEX
- Allow an index to be used for sorting the table
instead of filesort only if it is not disabled by
IGNORE INDEX.
When making a place to store field values at the start of each group
the real item (not the reference) must be used when deciding which column
to copy.
length.
When temporary field created for DATE(LEFT(column,8)) expression, max_length
value is taken from Item_date_typecast, and it is getting it from underlaid
Item_func_left and it's max_length is 8 in given expression. And all this
results in stripping last 2 digits.
To Item_date_typecast class added its own fix_length_and_dec() function
that sets max_length value to 10, which is proper for DATE field.
Added a test case for bug #11414.
sql_select.cc:
Fixed bug #11414: crash on Windows with some simple
GROUP BY queries.
It happened to an allocation of an array containing
0 Copy_field elements in setup_copy_fields.
The bug had been already fixed in 5.0.
Added a test case for bug #11295.
item_buff.cc:
Fixed bug #11295.
This a correction for the patch of bug #11088 that takes into
account a possible NULL values of the BLOB column.
Added a test case for bug #11385.
group_by.test:
Added a test case for bug #11385.
field.h:
Fixed bug #11385.
The bug was due to not defined method decimals for the class
Field_datetime.
- Print warning that says display width is not supported for datatype TIMESTAMP, if user tries to create a TIMESTAMP column with display width.
- Use display width for TIMESTAMP only in type_timestamp test to make sure warning is displayed correctly.
Added a test case for bug #8614.
sql_select.cc:
Fixed bug #8614.
SELECT DISTINCT ... GROUP BY 'const' must be equivalent to
SELECT ... GROUP BY 'const'.
Fixed bug #11088: a crash for queries with GROUP BY a BLOB column
+ COUNT(DISTINCT...) due to an attempt to allocate a too large
buffer for the BLOB field.
Now the size of the buffer is limited by max_sort_length.
group_by.test, group_by.result:
Added a test case for bug #11088.
Ensure that references in HAVING, ORDER BY or GROUP BY are calculated after fields in SELECT.
This will ensure that any reference to these has a valid value.
Generalized the code for split_sum_func()
In Item_ref::Item_ref set maybe_null (and other fields fix_fields sets) to be the
same as in (*ref), because Item_ref::fix_fields() will not be called. Previously
maybe_null was 0 always and this produced a bogus state where
maybe_null==0 && is_null() == true
which broke evaluation for some upper-level Items, like AND and OR.
