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MDEV-21104 Wrong result (extra rows and wrong values) with incremental BNLH

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This bug could affect multi-way join queries with embedded outer joins that
contained a conjunctive IS NULL predicate over a non-nullable column from
inner table of an outer join. The predicate could occur in WHERE condition
or in ON condition. Due to this bug a wrong result set could be returned by
the query. The bug manifested itself only when join buffers were employed
for join operations.

The problem appeared because
- a bug in the function JOIN_CACHE::get_match_flag_by_pos that not always
  returned proper match flags for embedding outer joins stored together
  with table rows put a join buffer.
- bug in the function JOIN_CACHE::join_matching_records that not always
  correctly determined that a row from the buffer could be skipped due
  to applied 'not_exists' optimization.
Example:
  SELECT * FROM t1 LEFT JOIN ((t2 LEFT JOIN t3 ON c = d) JOIN t4) ON b = e
    WHERE e IS NULL;

The patch introduces a new function that finds the match flag for a record
from join buffer specifying the buffer where this flag has to be found.
The function is called JOIN_CACHE::get_match_flag_by_pos_from_join_buffer().
Now this function rather than JOIN_CACHE::get_match_flag_by_pos() is used
in JOIN_CACHE::skip_if_matched() to check whether a record from the join
buffer must be ignored when extending the record by null complements.
Also the code of the function JOIN_CACHE::skip_if_not_needed_match() has
been changed. The function checks whether a record from the join buffer
still may produce some useful extensions.
Also some clarifying comments has been added.

Approved by monty@mariadb.com.
This commit is contained in:
Igor Babaev
2021-03-10 17:26:43 -08:00
parent 1af8558193
commit 90780bb5a9
4 changed files with 177 additions and 11 deletions
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11
sql/sql_join_cache.h
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@ -206,7 +206,9 @@ protected:
/*
This flag indicates that records written into the join buffer contain
a match flag field. The flag must be set by the init method.
a match flag field. The flag must be set by the init method.
Currently any implementation of the virtial init method calls
the function JOIN_CACHE::calc_record_fields() to set this flag.
*/
bool with_match_flag;
/*
@ -646,6 +648,13 @@ public:
/* Shall return the value of the match flag for the positioned record */
virtual enum Match_flag get_match_flag_by_pos(uchar *rec_ptr);
/*
Shall return the value of the match flag for the positioned record
from the join buffer attached to the specified table
*/
virtual enum Match_flag
get_match_flag_by_pos_from_join_buffer(uchar *rec_ptr, JOIN_TAB *tab);
/* Shall return the position of the current record */
virtual uchar *get_curr_rec() { return curr_rec_pos; }