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In MariaDB up to 10.11, the test_if_cheaper_ordering() code (that tries
to optimizer how GROUP BY is executed) assumes that if a table scan is used
then if there is any index usable by GROUP BY it will be used.
The reason MySQL 10.4 provides a better plan is because of two differences:
- Plans using 'ref' has a cost of 1/10 of what it should be (as a
protection against table scans). This is why 'ref' is used in 10.4
and not in 10.5.
- When 'ref' is used, then GROUP BY will not use an index for GROUP BY.
In MariaDB 10.5 the chosen plan is a table scan (as it calculated to be
faster) but as 'ref' is not used, the test_if_cheaper_ordering()
optimizer phase decides (as ref is not usd) to use an index for GROUP BY,
which has bad performance.
Description of fix:
- All new code is protected by the "optimizer_adjust_secondary_key_costs"
variable, which is now a bit map, and is only executed if the option
"disable_forced_index_in_group_by" set.
- Corrects GROUP BY handling in test_if_cheaper_ordering() by making
the choise of using and index with GROUP BY cost based instead of rule
based.
- Adds TIME_FOR_COMPARE to all costs, when using group by, to make
read_time, index_scan_time and range_cost comparable.
Other things:
- Made optimizer_adjust_secondary_key_costs a bit map (compatible with old
code).
Notes:
Current code ignores costs for the algorithm used when doing GROUP
BY on the first table:
- Create an in-memory temporary table for handling group by and doing a
filesort of the result file
We can probably in 10.6 continue to ignore this cost.
This patch should NOT be merged to 11.0 series (not needed in 11.0).
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