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MDEV-6768 Wrong result with aggregate with join with no result set

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When a query does implicit grouping and join operation produces an empty
result set, a NULL-complemented row combination is generated.
However, constant table fields still show non-NULL values.

What happens in the is that end_send_group() is called with a
const row but without any rows matching the WHERE clause.
This last part is shown by 'join->first_record' not being set.

This causes item->no_rows_in_result() to be called for all items to reset
all sum functions to their initial state. However fields are not set
to NULL.

The used fix is to produce NULL-complemented records for constant tables
as well. Also, reset the constant table's records back in case we're
in a subquery which may get re-executed.
An alternative fix would have item->no_rows_in_result() also work
with Item_field objects.

There is some other issues with the code:
- join->no_rows_in_result_called is used but never set.
- Tables that are used with group functions are not properly marked as
  maybe_null, which is required if the table rows should be regarded as
  null-complemented (not existing).
- The code that tries to detect if mixed_implicit_grouping should be set
  didn't take into account all usage of fields and sum functions.
- Item_func::restore_to_before_no_rows_in_result() called the wrong
  function.
- join->clear() does not use a table_map argument to clear_tables(),
  which caused it to ignore constant tables.
- unclear_tables() does not correctly restore status to what is
  was before clear_tables().

Main bug fix was to always use a table_map argument to clear_tables() and
always use join->clear() and clear_tables() together with unclear_tables().

Other fixes:
- Fixed Item_func::restore_to_before_no_rows_in_result()
- Set 'join->no_rows_in_result_called' when no_rows_in_result_set()
  is called.
- Removed not used argument from setup_end_select_func().
- More code comments
- Ensure that end_send_group() modifies the same fields as are in the
  result set.
- Changed return_zero_rows() to use pointers instead of references,
  similar to the rest of the code.

Reviewer: Sergei Petrunia <sergey@mariadb.com>
This commit is contained in:
Monty
2023-03-31 19:35:04 +03:00
parent c0adb05b30
commit 16258677b3
8 changed files with 397 additions and 107 deletions
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5
sql/sql_select.h
View File

@ -227,7 +227,7 @@ enum sj_strategy_enum
typedef enum_nested_loop_state
(*Next_select_func)(JOIN *, struct st_join_table *, bool);
Next_select_func setup_end_select_func(JOIN *join, JOIN_TAB *tab);
Next_select_func setup_end_select_func(JOIN *join);
int rr_sequential(READ_RECORD *info);
int rr_sequential_and_unpack(READ_RECORD *info);
Item *remove_pushed_top_conjuncts(THD *thd, Item *cond);
@ -1767,7 +1767,8 @@ public:
void join_free();
/** Cleanup this JOIN, possibly for reuse */
void cleanup(bool full);
void clear();
void clear(table_map *cleared_tables);
void inline clear_sum_funcs();
bool send_row_on_empty_set()
{
return (do_send_rows && implicit_grouping && !group_optimized_away &&