MariaDB 5.3.10 Changelog

Fix for Bug#12667154 SAME QUERY EXEC AS WHERE SUBQ GIVES DIFFERENT
 RESULTS ON IN() & NOT IN() COMP #3
. 
 This bug causes a wrong result in mysql-trunk when ICP is used
 and bad performance in mysql-5.5 and mysql-trunk.
. 
 Using the query from bug report to explain what happens and causes
 the wrong result from the query when ICP is enabled:
. 
 1. The t3 table contains four records. The outer query will read
 these and for each of these it will execute the subquery.
. 
 2. Before the first execution of the subquery it will be optimized. In
 this case the important is what happens to the first table t1:
 -make_join_select() will call the range optimizer which decides
 that t1 should be accessed using a range scan on the k1 index
 It creates a QUICK_RANGE_SELECT object for this.
 -As the last part of optimization the ICP code pushes the
 condition down to the storage engine for table t1 on the k1 index.
. 
 This produces the following information in the explain for this table:
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 2 DEPENDENT SUBQUERY t1 range k1 k1 5 NULL 3 Using index condition; Using filesort
. 
 Note the use of filesort.
. 
 3. The first execution of the subquery does (among other things) due
 to the need for sorting:
 a. Call create_sort_index() which again will call find_all_keys():
 b. find_all_keys() will read the required keys for all qualifying
 rows from the storage engine. To do this it checks if it has a
 quick-select for the table. It will use the quick-select for
 reading records. In this case it will read four records from the
 storage engine (based on the range criteria). The storage engine
 will evaluate the pushed index condition for each record.
 c. At the end of create_sort_index() there is code that cleans up a
 lot of stuff on the join tab. One of the things that is cleaned
 is the select object. The result of this is that the
 quick-select object created in make_join_select is deleted.
. 
 4. The second execution of the subquery does the same as the first but
 the result is different:
 a. Call create_sort_index() which again will call find_all_keys()
 (same as for the first execution)
 b. find_all_keys() will read the keys from the storage engine. To
 do this it checks if it has a quick-select for the table. Now
 there is NO quick-select object(!) (since it was deleted in
 step 3c). So find_all_keys defaults to read the table using a
 table scan instead. So instead of reading the four relevant records
 in the range it reads the entire table (6 records). It then
 evaluates the table's condition (and here it goes wrong). Since
 the entire condition has been pushed down to the storage engine
 using ICP all 6 records qualify. (Note that the storage engine
 will not evaluate the pushed index condition in this case since
 it was pushed for the k1 index and now we do a table scan
 without any index being used).
 The result is that here we return six qualifying key values
 instead of four due to not evaluating the table's condition.
 c. As above.
. 
 5. The two last execution of the subquery will also produce wrong results
 for the same reason.
. 
 Summary: The problem occurs due to all but the first executions of the
 subquery is done as a table scan without evaluating the table's
 condition (which is pushed to the storage engine on a different
 index). This is caused by the create_sort_index() function deleting
 the quick-select object that should have been used for executing the
 subquery as a range scan.
. 
 Note that this bug in addition to causing wrong results also can
 result in bad performance due to executing the subquery using a table
 scan instead of a range scan. This is an issue in MySQL 5.5.
. 
 The fix for this problem is to avoid that the Quick-select-object that
 the optimizer created is deleted when create_sort_index() is doing
 clean-up of the join-tab. This will ensure that the quick-select
 object and the corresponding pushed index condition will be available
 and used by all following executions of the subquery.